Combustion chamber pressure of hybrid rocket motor was estimated to be proportional to its oxidizer mass flow rate under the conditions of short work duration of less than 2s or minor variation of grain port diameter within 10%. The conclusion was deduced from the exponential relation between solid fuel regression rate of hybrid rocket motor and corresponding oxidizer mass flux. The oxidizer mass flow rate during motor throttling or shutdown was calculated through pressure curve from motor test and obtain the fuel mass consumed consequently. By eliminating the mass consumed during motor shutdown

the overestimated regression rate calculated from motor test through end point average method was improved. Regression rates of two sets of propellant were calculated through the improved method from test results. The rates reduce about 20% and the results between different motor sizes agree with the scale effect within an error of 5%. It is thusreasonable to apply the regression rate from small scale motor to the design of large scale motor design.