One-dimensional thermodynamic mixed layer model to simulate variations of meteorological variables within the planetary boundary layer has been developed in this study. This model consists of 2 prognostic equations, which can predict the variations of potential temperature and mixing ratio and several diagnostic equations. Physics within the surface and mixed layers has been considered seperately in the model. For the variations of the model, its result has been analysed and compared with observated data over the Dukyang Bay for one day, July 23, 1992. The simulated height of mixed layer is comparable to the observation and the variations of temperature and mixing ratio in the mixed layer are also reasonably simulated. Those imply that the model responds appropriately with given boundary conditions in spite of its simplified assumptions applied to the model and insufficient boundary and initial conditions.