A non-destructive method based on measurements of ultrasonic wave velocities and Young's modulus is proposed for quantification of creep cavitation in silicon nitride. Tensile creep tests of silicon nitride were conducted at 1400 degreesC in air and the tests were periodically interrupted to measure the longitudinal and transverse ultrasonic wave velocities and Young's modulus. The velocities and Young's modulus decreased linearly with tensile creep strain. The volume fraction of cavities was estimated from the values of the ultrasonic wave velocities and Young's modulus, and compared with the cavity volume predicted from tensile creep strain. The dependence of Young's modulus on volume fraction of cavities is discussed.