The temperature dependence of electron mobility on 5 undoped LPE-GaAs samples has been measured. It was found that the experimental curves cross each other and this is difficult to explain by only one extrinsic scattering mechanism (ionized impurity scattering). Therefore, it's necessary to assume another extrinsic scattering mechanism, so called mobility killer. Because the samples we used are rather pure, the central-cell scattering suggested by Stringfellow et al. can certainly be neglected. The effect of illumination on the electron concentration and mobility at 77 K has been studied and it is considered that the nature of mobility killer is probably the spacecharge regior caused by the microscopic inhomogeneities in samples.