A motor task performance and imagery show several parallel characteristics. In this study, the effect of a voluntary speed change in mental images of a complex difficult sequence of karate techniques on subsequent motor performance was assessed. 24 male karate volunteers (mean age: 24.58±5.27 years) who were well skilled at performing the sequence movements participated in the pretest. During the training period, they imagined the motor task with normal, faster, or slower speeds in the real time, fast and slow groups respectively. The post-test was identical to the pretest. The data were analyzed by one-way variance analysis and t tests. The results showed no time difference between motor imagery duration and physical performance in real time group (P>0.05) while imagery speed decrease in slow group was not significant when compared with their own physical performance in the pretest (P>0.05). Fast group accelerated their mental imagery in the training period (P<0.05). The comparison between pretest and post-test by paired t test revealed that karate athletes improved the time of their performance after the real time and fast imagery (P<0.05). However, slow imagery had no effect on post-test performance (P>0.05). Also, independent t test results showed no significant difference between the effect of real time and the fast imagery (P>0.05). It seems that mental imagery in real speed of action and fast imagery had the advantage to improve the performance duration compared with the slower imagery. In conclusion, mental image speed is an important factor in motor imagery and it is suggested that it should be controlled by the athletes.