In this paper, the theory of apparent permeability and effective permeability of the core is described. The apparent permeability with coil in different positions of the core is investigated and simulated by HFSS. The best location of the coil is determined according to the curves of the apparent permeability. The relationship between the effective permeability and aspect ratio of the core is investigated and compared with the theoretical derivation, verifying the accuracy of theoretical analysis. Specification of core in shape is solved. Core losses are further discussed. Quantitative analysis of core losses is obtained.