Abstract: This paper describes the drilling fluid systems and maintenance programs for medium inclined directional well with long open hole, and it focuses on drilling fluid lubricity and anti-sticking property, hole cleaning capability and wellbore stability control. Drilling fluid performance in four wells are evaluated. The drilling fluid technology has been successfully used in a directional well with total measurement depth (TMD) of 3625m, hole deviation of 49°, horizontal displacement of 1229m, and 3224.5m open hole, in which 2292m is in the inclined section.