Abstract: Borehole instability often occurs in tuff formation drilling in Block C2 of Myanmar.We analyzed the characteristics and mineral components of the formation systematically.On the basis of the drilling fluid previously used in this area,a strong inhibition and plugging drilling fluid system was developed.Field tests show the performance of the new drilling fluid is for superior to the one previously used.Borehole collapse accidents have been greatly reduced and tripping speed improved significantly.In addition,borehole is more regular and drilling cycle is shortened.The new drilling fluid system can meet the requirements of quick and safe drilling,and borehole instability problems in this area can be solved effectively.