The performances of timber frames with loose connections under lateral load is studied in this paper with an analytical friction-slip-contact model of the mortise-tenon (MT) connection with gaps where different contact states between the mortise and tenon are described. The nonlinear contact behavior within the joint is modelled via an analytically formulated hybrid finite element with a MT connection or a column foot (CF) connection. The mechanical behavior of the connection with different horizontal and vertical gaps is further studied. The load–displacement curves at the top of the frame model with hybrid finite elements are compared with those from the solid element model in ABAQUS. They are matching each other very well. Parametric study with different height and length of tenon on the lateral resistance of the frame is also conducted. The gap in the MT connection is found to be the major source of weakness in the lateral resistance of the frame. Results obtained may serve as reference for the maintenance of heritage timber structures in China.