The CoX/Pt multilayer (X here represents the doping element to form grain boundary) is considered as the potential future medium owing to its high interfacial perpendicular anisotropy [1] utilized in magnetic assisted magnetic recording (MAMR) [2]. Meanwhile, microwave assisted reversal in Co/ Pt and Co/Pd has been reported in experiments [3,4]. The magnetic property and its writability of the CoX/Pt medium with various anisotropy energies Ku are simulated by micromagnetics. The CoX/Pt media are modeled with the Voronoi polycrystalline structure built on a regular mesh, as seen in Fig.1. A micromagnetic cell is 1nm×1nm, with 0.4 nm thickness in CoX layer. The magnetic parameters of the two phases inside crystal grains and at grain boundary are treated separately.