Mi lethek merupakan makanan khas Yogyakarta yang terbuat dari tepung tapioka dan tepung gaplek. Mi lethek memiliki kekurangan dalam karakteristik fisik yaitu cooking loss yang tinggi. Penggunaan tepung dengan kadar amilosa tinggi hasil modifikasi multisiklus autoklaf-pendinginan dan penambahan guar gum dalam pembuatan mi lethek diharapkan mengurangi cooking loss mi. Tujuan dari penelitian adalah untuk menentukan pengaruh jumlah siklus autoklaf-pendinginan terhadap karakteristik fisik dan kimia tepung tapioka dan tepung gaplek, menentukan jumlah siklus autoklaf-pendinginan terbaik berdasarkan kadar amilosa tertinggi; menentukan pengaruh rasio tepung tapioka:tepung gaplek modifikasi dan konsentrasi guar gum terhadap karakteristik mi lethek; memilih kombinasi perlakuan terbaik berdasarkan cooking loss, tekstur, dan karakteristik sensori mi lethek. Modifikasi tepung tapioka dan gaplek dilakukan dengan autoklaf-pendinginan 1 siklus, 2 siklus, dan 3 siklus. Proses pemanasan autoklaf dilakukan dengan suhu 121oC selama 30 menit, kemudian didinginkan dalam suhu ruang selama 45 menit, dan proses pendinginan dilakukan dengan suhu 4oC selama 24 jam. Mi lethek dibuat dengan variasi rasio tepung tapioka dan gaplek hasil modifikasi (25:75, 50:50, dan 75:25) serta penambahan guar gum (0%; 0,5%; 1%; 1,5%). Hasil penelitian menunjukkan bahwa metode autoklaf-pendinginan 2 siklus menghasilkan kadar amilosa tertinggi pada tepung tapioka yaitu 49,15±6,41% dan autoklaf-pendinginan 3 siklus menghasilkan kadar amilosa tertinggi pada tepung gaplek yaitu 72,04±4,01%. Semakin banyak penambahan tepung tapioka hasil modifikasi akan menghasilkan mi dengan cooking loss dan hardness yang rendah. Penambahan konsentrasi guar gum juga menurunkan cooking loss dan hardness. Mi lethek dengan rasio tepung tapioka modifikasi autoklaf-pendinginan 2 siklus dan tepung gaplek autoklaf-pendinginan 3 siklus 75:25, penambahan guar gum 1% merupakan mi lethek formulasi terbaik./Lethek noodle is Yogyakarta’s traditional food made from tapioca and cassava flours. Lethek noodle is known for its high cooking loss. Flours with high amylose obtained from multi-cycle autoclaving-cooling modification and addition of guar gum in lethek noodle making is expected to reduce cooking loss. The purpose of this research were to determine the effects of the number of autoclaving-cooling cycles on the physical and chemical characteristic of tapioca and cassava flours; determine the best number of autoclaving-cooling cycles based on the highest amylose content of tapioca and cassava flours; determine the effects of the ratio of modified tapioca to cassava flours and the concentration of guar gum added to the characteristics of lethek noodle; and determine best treatment combination based on cooking loss, texture, and sensory characteristics of lethek noodle. Modification of tapioca and cassava flours was done by 1, 2, and 3 cycles of autoclaving-cooling. Autoclaving was done in temperature of 121oC for 30 minutes, then cooled in room temperature for 45 minutes and cooled in 4oC for 24 hours. Lethek noodle is made with a variety of ratio between modified tapioca to cassava flours (25:75, 50:50, and 75:25) and the addition of guar gum (0%; 0.5%; 1%; 1.5%). The results showed that 2 cycles of autoclaving-cooling method produced the highest amylose content in tapioca flour which is 49.15±6.41%, meanwhile 3 cycles of autoclaving-cooling produced the highest amylose content in cassava flour which is 72.04±4.01%. The addition of modified tapioca flour resulted in noodle with low cooking loss and hardness. The higher concentration of guar gum added also reduced cooking loss and hardness of lethek noodle. Lethek noodle made of tapioca flour modified by 2 cycles of autoclaving-cooling and cassava flour modified by 3 cycles autoclaving-cooling with the ratio of 75:25 and the addition of 1% guar gum is the best formulation of lethek noodle.