The lock-up clutch pressure observer is designed based on input-to-state stability（ ISS）.Considering the modeling error and additional disturbance,the ISS theory is introduced to theory to adjust the estimator parameter and ensure the system stability,and taking the uncertainty of model as disturbance inputs and the estimator is designed to ensure the error system is input- to- state stable.Based on the AMESim,the transmission model of automatic transmission is established.The designed observer is realized in the MATLAB/Simulink,and the joint simulation with the transmission model is carried out to verify the correctness of designed lock- up pressure observer of lock- up clutch.Comparing with the sliding mode observer,the designed pressure observer has the better performance.