Abstract: Although positive displacement down-hole motor(PDM)is commonly used in directional drilling at present,not much effort has been made in research on the optimization methods for drilling hydraulic parameters when using a PDM.An optimization model in drilling fluid rate and drill bit nozzle pressure-drop selection has been established by mathematical programming method while taking PDM’s recommended operating parameters as constrain conditions.The research results show that hydraulic parameters should be optimized within PDM’s recommended operating parameters while taking maximum total hydraulic power designated to drill bit and PDM as the target.To obtain optimum drilling hydraulic parameters,the PDM’s working pressure-drop should be within the recommended working pressure-drop while bit nozzle pressure-drop is close to the maximum of that transmission shaft of the down hole motor allows to.