Exponential distribution is widely applied to describe product life in reliability engineering. Correspondingly,product failure rate is a constant (not changing with the service time of the product). Nevertheless, only the products with special property or under particular load condition have exponentially distributed life and constant failure rate. Unrealistic hypothesis of exponentially distributed life will lead to serious error in reliability and failure rate analysis results. In the situations that component life follows exponential distribution, to assume component failures being independent of each other will mislead system reliability evaluation. The the property of product failure rate was analyzed and inferred from the aspects of product strength performance and load environment.The conditions for product failure rate to follow exponential distribution were revealed, and product failure dependency issues in condition of life following exponential distribution were explained.