The laminar flame speed is one of the most important parameters of a combustible mixture. Recently, it became clear that the accurate determination of laminar flame speed at high pressures and temperatures is extremely important to the development and validation of kinetic mechanisms for gasoline and diesel surrogate fuels and alternative fuels. In this study, the effect of flame stretch on the accurate determination of laminar flame speed at normal and elevated pressures using spherical bombs are studied theoretically and numerically. The result shows that for mixtures with Lewis numbers greatly deviating from unity, the flame stretch has great impacts on the accuracy of the measured flame speed. An analytical expression for the stretch effect is obtained, and a method to obtain the Stretch-Corrected Flame Speed (SCFS) at elevated pressures and temperatures is developed. The theoretical results are validated by numerical simulations of expanding flames in a spherical bomb using detailed chemistry for hydrogen/air, methane/air and propane/air mixtures. It is demonstrated that the SCFS method can greatly improve the accuracy of the flame speed measurements.