Factors affecting the shape of failure rate curve are analyzed, and four elements including load, strength, degradation and lifetime index (namely, the number of load application and time) are proposed for calculating the failure rate of components. Taking the number of load application and time as the life index, respectively, the failure rate models of components consisting of four elements is developed, which can embody the effect of parameters of load, strength, degradation and life. Then, the rules that the reliability and failure rate changes as life index are studied. The result shows that the failure rate curves are partially or whole bathtub-shaped. For different parameters of load, strength and strength degradation, the curves of reliability and failure rate of components changing with time present different shapes. The models derived in this paper can calculate the failure rate of components as long as the parameters of load, strength and degradation and life index are known.