Abstract: Survivability of a gear transmission system in attack helicopter after Loss-Of-Lubrication (LOL) is greatly dependent on the transient temperature fields of the systems.A helicopter gear transmission system was researched.A method is put forward to calculate the transient temperature fields of this system after LOL and the models for power losses and heat transfer (convection) coefficients are established.In these models,the time-variance characteristics of thermal physical parameters,such as viscosity,thermal conductivity are considered.A computer program is worked out for calculating the transient temperature fields of the system on the basis of these models.In the computation,the calculational results of the steady state temperature field in the transmission system are used as initial values of the transient temperature fields and the time steps are properly controlled so as to prevent "data oscillation".The transient temperature fields of the gear transmission system are obtained from the computation and can serve for predicting the survivability of the gear transmission system.