The brittle fracture initiation characteristics can be evaluated by measuring the crack opening displacement (C. O. D.). Especially, it is very difficult to analyze the brittle fracture phenomenon of welded structures by computation, and the measured C. O. D. in the vicinity of crack tip is significant. The authors developed newly a measuring method of crack opening displacement by means of the measurement of change in electrostatic capacitance caused by the crack opening. A sheet of foil strain gage as an electrode inserted in the slit works effectively for the measurement of change in clearance of slit. By adopting this new method, effects of width of test specimen and slit were studied from viewpoint of C. O. D. experimentally as well as theoretically.