The research aims to study the effect of Specific 10-20-30 Interval Training on developing aerobic endurance and Repeated Change Of Direction Speed Ability variables and specific anaerobic capacity for juniors squash under 13 years, The researchers use experimental methodology with two groups design (experimental and control) and pre-post measurements, The research sample was chosen in Purposive method, 16 juniors squash registered in the Egyptian Squash Federation 2016-2017 were divided as follows (8) juniors of the experimental group, (8) juniors of the control group, The researchers add Specific 10-20-30 Interval training program ( 8 weeks by three training units per week) as additional training units for the basic program for the experimental group, the results of this study were the Specific 10-20-30 Interval has a positive effect on developing aerobic endurance ( maximum oxygen consumption - maximal aerobic speed - time limit to exhaustion ) and Repeated Change Of Direction Speed Ability variables and specific anaerobic capacity for juniors squash.