Abstract: The moisture expansion performance is one of the key properties of structural materials with high dimensional stability. And the coefficient of moisture expansion (CME) is the main index to measure the moisture expansion properties of materials. However, no standard measurement method has been established for the CME of composites. In this paper, the CME of cyanate based carbon fiber composites was studied by experiments. Firstly, a comparative analysis was carried out by the two static measurement methods of moisture by absorption after drying and dehumidification after moisture saturation. The results show that the test efficiency of dehumidification after moisture saturation was higher. Subsequently, the CMEs of M55J/cyanate and T700/cyanate carbon fiber plies were measured and analyzed by the method of dehumidification after moisture saturation. The test results indicate that specimens with thinner unidirectional ply thickness of the same material have lower CMEs. M55J/cyanate has a lower CME than T700/cyanate among multi-directional ply specimens with the same ply-angle and ply-thickness. The test specimens using quasi isotropic ply 0°/+45°/-45°/90°_S have lower CMEs compared to (0/+60/-60_S)₂. The research above may provide a reference for the study of the CME properties of cyanate based carbon fiber composites and the analysis of the moisture release deformation of dimensional stability structures.