Abstract: In view of some problems about weighting agent in ultra-high density drilling fluid,such as large dosage,scarce supply,expensive prices,high hardness,drilling tools wear,toxicity,etc.,weighting agents,most widely used barite and MicroMax developed abroad,were selected to study their influence on drilling fluid’s properties.Their microscopic shape and size distribution were analyzed and then two kinds of ultra-high density drilling fluid (ρ=2.85-2.90 kg/L) were prepared with barite and the mixture of barite and MicroMAX as weighting materials to evaluate their rheology,filtration and sedimentation stability.After high temperature aging,the test results showed that high purity barite can make ultra-high density drilling fluid with good rheology,suspension stability and low filtration at high temperature and pressure.Whereas the ultra-high density drilling fluid,using mixed barite and MicroMAX of higher density and smaller grain size as weighting agent,showed reduction in viscosity and dynamic shear force,improvement in rheology,but increased filtration at high temperature and high pressure.Taking drilling fluid performance,economy and practicability into comprehensive consideration,it is suggested that the cost effective barite alone be used as weighting agent as far as possible.The ultra-high density drilling fluid based on barite with densities of 2.75-2.89 kg/L was tested in Well Guanshen 1 in Guandu Area,which was safely drilled for 745 m.