A theory of alloy scattering of two‐dimensional electron gas in ternary semiconductors is developed by assuming spherically symmetric square scattering potential, randomly distributed between two kinds of alloy sites. The theory predicts a temperature independent mobility, in agreement with the experimental results for In0.53Ga0.47As. The calculated value at 4.2 K comes close to the experimental value, in which the effect of impurity scattering has been reduced by using an undoped spacer layer. It is concluded that the study of two‐dimensional transport in the devices may lead to a correct estimation of the alloy scattering potential.