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The strength of plastic encapsulants is analyzed from the viewpoint of crack propagation. With a preexisting crack length a and a specific number of applied load cycles N, fatigue crack propagation rates da/dN of the encapsulants were measured with laboratory test specimens. It was found that the da/dN of encapsulants can be expressed as functions of the stress intensity factor range K. The crack propagation behavior in the package was estimated from the data of da/dN and K at the lowest temperature of the test cycles. Reasonable correlation is found between the estimated crack propagation behavior and the observed one. The applicability of fracture mechanics to the package Cracking problem is demonstrated.