Surface tracking on organic solid insulators is one of the main reasons for failure in high voltage systems. Due to various factors, such as humidity, pollution, ice load, increase in local voltage, etc., it is hard to estime the lifetime of an insulator. For many years, Weibull statistics have been widely used and accepted as a successful mathematical method of predicting the remaining lifetime of an insulating material. The basic reliability function can perform well for predefined conditions. However, it might be insufficient in lifetime prediction in multi-variable conditions. In this research, a model based on improved Weibull statistics is proposed for estimating the break- down time of polymeric insulation material. By using appropriate parameters, this improved model can estimate the remaining lifetime within a reasonable accuracy in varying external conditions such as voltages, contaminant flow rates and applied load. To determine the model parameters, several tests are performed according to the IEC 587 accelerated inclined plane tracking test method by investigating the breakdown times of polyesters under various external conditions generated artificially in the laboratory. The calculated results are found to be in good agreement with the experimental data.