Abstract: The diversity gain of an X-type polarization diversity system is heavily affected by mutual coupling (MC) effect. In order to study the relationship between the antenna diversity gain and the cross-angle of X-type antenna array in the presence of MC, the mathematical expressions of mutual impedance, spatial correlation and average receive power ratio for X-type antenna array are derived firstly based on basic antenna theory; then the polarization diversity gain changing with the cross-angle of X-type antenna array is analyzed indetail through the above derivations. Both theoretical analysis and numerical simulations show that the range of the cross-angle formed by the X-type antenna layout to guarantee the final diversity gain with MC over that without MC increases with the increment of the cross polarization discrimination (XPD) of the incident waves. Due to higher XPD level in actual communication environment, the research results are quite valuable for compact MIMO system design.