Hybrid rocket solid fuel grain regression rate is one of the significant values to accurately determine in the hybrid rocket motor (HRM) design parameters and performance prediction. The comprehensive method to measure and evaluate HRM solid fuel regression rate was investigated theoretically and experimentally .The mathematical model has been introduced and a computer program was built to describe HRM regression rate. A series of experimental tests using a small scale HRM employs Polyethylene (PE) and aluminized PE as solid fuel (up to 15% by mass metal powder additive) with different initial port diameter of fuel grain (5, 20 and 28mm) and gaseous oxygen (GO2) as oxidizer to evaluate the HRM regression rate performance. The measured experimental data are compared with results obtained from theoretical computation. The comparison shows good agreement, which proves the validity of the developed program. Regression rate of hybrid fuel grain was enhanced by addition of Al powder compared to basic configuration (0% Al) and it was found that it is more significant to use Al powder additive increased up to 7.5% by mass