Based on the void growth and damage mechanics, the rupture time t//r is derived as a function of the creep exponent n and the reference stress sigma //0 and the corresponding time t//0, for materials obeying exponential and power law creep. The analysis is extended to obtain parameters for prediction of long time behaviour. For materials which obey a simple power law creep, the parameter is shown as P equals log t//r plus (n minus n//r) log (1 minus D//c) and for materials which obey an exponential function the parameter is given as P equals log t//r plus (n//r minus n)D//c where n//r is the reference creep exponent and D//c is the critical size of the void at fracture, which is related to the applied stress sigma as D//c equals 1 minus sigma / sigma //0.