Research objective: This research aimed at : Identify the effect of mental imagery on developing some mental, psychological, and physical abilities and on Bhavsar skill's dynamic performance on parallel bars for juniors under-15 years Research hypotheses: There are statistically significant differences between pre- and post measurements for experimental group in measurement of mental imagery, some physical abilities and Bhavsar skill's performance level on parallel bars for juniors under-15 years There are statistically significant differences between pre- and post measurements for control  group in measurement of mental imagery, some physical abilities and Bhavsar skill's performance level on parallel bars for juniors under-15 years There are statistically significant differences between post measurements for experimental and control group in measurement of mental imagery, some physical abilities and Bhavsar skill's performance level on parallel bars for juniors under-15 years Results: Within research aim and followed procedures, following could be concluded: The experimental group surpassed the control group in in-between and post measurements of Bhavsarskill performance level on parallel bars for junior gymnasts under 15 years. The proposed mental imagery program significantly had a positive effect on developing some psychological skills: Relaxation - mental imagery aspects (vision - auditory – locomotor- emotional – image control in the pictures) for experimental group with improvement percentages greater than control group, where relaxation improvement percentage for control group was (67%) compared to (29%) for control group, and mental imagery for experimental group was (40.15%) compared to (9.27%) for control group. The proposed mental imagery program significantly had a positive effect on developing Bhavsarskill performance level for experimental group with improvement percentage equal to (187.5%) for experimental group compared to (91.75%) for control group.