The research aims to study the effect of Modified Sprint Interval Training on developing some physical and physiological variables 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, 12 juniors squash registered in the Egyptian Squash Federation 2018-2019 were divided as follows (6) juniors of the experimental group, (6) juniors of the control group, The researchers add modified sprint 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 modified sprint interval training has a positive effect on developing some physical variables ( sprint - acceleration - muscular power - repeated change of direction speed ability ) and physiological variables ( maximum oxygen consumption - maximal aerobic speed - time limit to exhaustion - specific anaerobic capacity ) for juniors squash