"Accelerated life testing" involves acceleration of failures with the single purpose of the "quantification of the life characteristics of the product at normal use conditions." This book is solely concerned with this type of accelerated life testing. During the last few decades, the statistician and other researchers had been concentrated about life testing using probability models. The parameters involved in the models usually had been estimated by method of moment and maximum likelihood. The literature cited above indicated about the life testing using probability distributions. The present investigation was undertaken by taking the probability density function of exponential and Weibull distribution to determine general equation for failure rate function and failure time distribution, and also the study of mean life of the system through these distributions.The failure rate function of exponential distribution is . Thus, the failure rate function for the exponential distribution is constant. The parameter is often referred to as the rate of the distribution, which is also known as memory less property.