Dual-in-line packages (DIPs) were formed from epoxy molding compounds with various physical properties using a transfer molding machine. The compounds were prepared by changing kinds and amounts of additives and addition methods. The thermal shock test was carried out by the following procedures. The plastic package was soaked alternately in liquid nitrogen (−196°C) and in liquid solder (250°C) in the cycle of 140 s. The packages were intermittently subjected to optical microscopic inspection for crack initiation. The median life to crack initiation is defined to be the number of the cycles when half of the specimens exhibited crack initiation. Glass transition temperature, coefficient of linear expansion, and flexural modulus were measured to calculate thermal stress in plastic packages. According to the linear fracture mechanics, the following expression was obtained among the median life N, thermal stress σt and strength σb. N =/σ We found the linear relation between logarithm of Nσ and logarithm of σb/σ t for various packages, and obtained that the value of C and m are estimated as 3 × 104 MPa2 and 5.5, respectively. Therefore, the median life can be predicted from the glass transition temperature, coefficient of linear expansion, flexural modulus, and strength of plastic materials for packages.