Abstract Thermoelectric power of two-dimensional electron gas has been calculated by using the formula derived from the Boltzmann equation assuming constant density-of-states and relevant scattering mechanisms. The values increase linearly with temperature. The present theory is applicable to the high mobility samples, in which the additional contribution due to phonon drag may account for higher experimental values. For low mobility GaAs samples, experimental values are lower and in InGaAs the values decrease with temperature. We have considered the subband energy to be a Gaussian variable caused by long-range potential fluctuations and found that the calculated values come closer to the experimental data the larger the value of the broadening parameter is.