The present study was conducted to investigate the effect of decision-making exercises on cognitive flexibility and service performance among semi-skilled and beginner volleyball players. This study is an applied and semi-experimental study with a pre-test and post-test design. The participants of study were 40 semi-skilled and beginner male volleyball players from Tabriz University who were selected purposively and placed in equal numbers (10 people) into four groups: semi-skilled with decision-making practice, semi-skilled control, beginner with decision-making practice and beginner control. The data collection tools were Dennis and Vander Wal's Cognitive Flexibility Inventory (2010) and the AAHPERD Service Test (1969). In the present study, the practice groups performed decision-making exercises for 18 sessions (3 weeks, 6 sessions per week). Data analysis was performed using descriptive analysis of anthropometric data, Shapiro-Wilk test, multivariate analysis of covariance test and Tukey post hoc test in SPSS23 software at a significance level of 0.05. The results showed that decision-making exercises have a significant effect on both cognitive flexibility variables (sig=0.001) and volleyball service performance (sig=0.001) in both groups of beginners and semi-skilled individuals. The results of the study also showed that decision-making exercises do not have a different effect on cognitive flexibility between semi-skilled and beginner volleyball players (sig=0.979), but this effect on the service performance of semi-skilled and beginner volleyball players is different (sig=0.001). Therefore, decision-making exercises can prepare the thinking and consequently the behavior of beginner and semi-skilled volleyball players to respond adaptively to changes and difficult environmental conditions and improve their performance.