Abstract: The stress field，saturated stress，stress transfer efficiency and effective length in carbon nanotubes (CNTs) reinforced polymer composites were analyzed based on the three-phase concentric cylindrical shell model along with shear-lag arguments. The thickness for interphase is comparable with the diameter of CNTs(0.1~100nm)，in the CNTs reinforced polymer composites. The interphase region should be included in the three-phase concentric cylindrical shell model to analyze the stress in CNTs reinforced polymer composites. The stress transfer efficiency and the CNTs saturated stress decrease monotonically with increasing the interphase thickness，but the converse is true for the CNTs effective length. The aspect ratio of CNTs influences the stress transfer efficiency and effective length in the CNTs reinforced polymer composites significantly when the aspect ratio is small.