Cognitive intelligence is the intelligence possessed by human to process and make a decision using common sense. According to the experts, cognitive intelligence is divided into five parts namely, memory, speed, attention, problem solving, and flexibility. Each of these parts of the brain represents the performance of a particular sector. Cognitive intelligence is very critical in times of human growth, especially during their childhood. Experts say that specific approach is needed to improve any children’s cognitive intelligence. To implement this approach, a game called ‘Onion Boy’ is developed to serve the purpose with primary focus is on speed and flexibility aspects. In evaluating the effectiveness of the application, a well-known instrument test called Stroop Effect is used before and after any player plays the game. This test which involves colors and texts is designed to measure the reaction time of human in responding to the interference. ‘Onion Boy’ reflects partially the characteristic of this benchmark. From a series of test conducted, the result shows that statistically the two cognitive aspects of each player after playing the game five times has improved. The flexibility aspect of all players are at the highest rank; similarly, though not at highest rank, the speed aspect also improves. For further development one may consider how to improve these aspects significantly, e.g. using various objects and more colors.