In order to avoid the complicated influence of grain boundaries on the resistivity in the investigation of the "intrinsic" magnetoresistance (MR) effect, the magnetic and magnetotransport properties of La0.67Ca0.33MnO3 and La0.67Ca0.33Mn0.96Fe0.04O3 single crystals were investigated. Owing to the absence of grain boundaries in the single crystals, MR ratio under 40× 105A/m is about two orders of magnitude larger than that in polycrystalline counterparts.A further enhancement of MR ratio from 3400% to 17600% was achieved by a few percent of Fe doping. On the basis of the small polaron model, the activation energy derived from fitting the resistivity above Tc was found to increase upon introducing Fe. A connection between the strong localization of electrons arising from magnetic polarons and the enhancement of MR was observed in the single crystals.