A new method is developed for determining rock strength while drilling with a tri-cone roller bit. The technique employs the well-known ＇＇drill-off＇＇ test, using commonly measured drilling data, and does not require knowledge of drill bit parameters. The method was evaluated with drill-off test darn collected from the Gas Research Institute＇s SFE (staged field experiment) well no. 4 and compared with calculated in-situ compressive rock strengths provided by Hareland (1992). The results indicate that the proposed approach produces reasonable results and is easily implemented.