The purpose of this study was to explore the effectiveness of the number of practice trials in determining the degree of task complexity. Sixty subjects according to the types of tasks (A-B) and receiving feedback (100%, omitted, self-control) were randomly divided into six groups.All of the subjects practiced 100 trials tasks in each day on first two days. Retention tests were performed during the second and third day and reacquisition tests were performed on the third day. Using ANOVA with repeated measures showed found that,all of the subjects improved across practice trials on both days but this improvement in B task was significantly better than A task on the second day. Subjects were significantly better in the second retention and reacquisition tests compare with first one’s, in addition subjects that practiced B task were better than those that practiced A task in the second retention test (p<0.05). Results indicate that A task was more complex than B task, and the optimal practice trials is required to understanding which task is complex.