The force mechanism was analyzed and the moment-rotation calculation formulas were deduced for the straight mortise-tenon joints.Good agreement between the predictions and test results was observed.Based on the moment-rotation calculation formulas,the factors affecting the rotational stiffness of the straight mortise-tenon joints were analyzed.The results show that:the rotational moment and initial rotational stiffness are increased significantly with the increase of the length of a tenon when the ratio of the length to the column diameter is less than 1;the rotational moment and initial rotational stiffness are also improved with the increase of the width of a tenon;the rotation moment of the joint is increased to a certain extent with the increase of the friction coefficients,but the initial rotational stiffness is affected little.The conclusins are helpful in the seismic performance assessment and reinforcement of the traditional wooden buildings.