The reliability-growth testing is an inevitable project item for increasing the reliability of product in the process of developing products. The Gompertz model is usually used to estimate the data of the reliability-growth for success-and-failure test, but it shows poor fitness for S-shaped growth data. A least square method of nonlinear regression for modified Gompertz model parameter estimation is brought forward according to Gauss-Newton least square method and multiple linear regression least square method, and the fitness for the modified Gompertz model can be proved by using the data of test.