The purpose of this study was to examine improvements in tactical decision-making ability achieved by participating in basketball games with numerically uneven sides (3 on 2). The research focused on in-game decision-making as an important component of the learning of the basketball tactics in physical education classes. Three elementary school basketball classes consisting of two grade 5 classes and 1 grade 6 class were examined. The 82 class participants (boys=38, girls=44) received a total of 11 hours instruction. A physical education researcher advised the elementary school teacher (1) how to teach the principles of offensive decision-making in basketball and (2) how to facilitate improvements in tactical decision-making ability within the context of numerically uneven sides. In order to analyze children's decision-making ability in the games, all game situations were filmed with digital video cameras and the Game Performance Assessment Instrument (GPAI) was used. The main findings are summarized as follows : 1) In all cases of shooting, passing and ball-keeping, the rate of appropriate decision-making, according to the data obtained from later units (classes 10 and 11), showed a significant (p>.05) increase when compared with data obtained from earlier units (classes 2 and 3). 2) Results indicated that the appropriate ball-keeping performances, represented as a percentage, were lower than both those of shooting and passing throughout the duration of the study. 3) Appropriate decision-making, when represented as a percentage, improved with learning experience in numerically uneven sides. These results support the hypothesis that the decision-making ability of children improves with learning experience by playing basketball games with numerically uneven sides.