Abstract: Based on the analysis on the force mechanism of dovetail joints, their rotational moment was analyzed and a moment-rotation calculation formula was deduced. The existed test approved that the formula is accurate and the calculated result agrees well with the test result. According to the moment-rotation calculation formula, the factors affecting the rotational stiffness of dovetail joints were analyzed. The results show that: the rotational moment and initial rotational stiffness are increased as the increase of friction coefficients; the increase amplitude of initial rotational stiffness is reduced when the friction coefficient is more than 0.5; the rotational moment and initial rotational stiffness are improved with the increase of the length and angle of a tenon; the rofational moment is improved with the increase of the height of a tenon, but the initial rotational stiffness very little. The conclusions could provide a theoretical basis for the mechanics analysis of ancient timber structure buildings.