Thermal stress in silicon chips encapsulated in IC plastic packages is discussed. A stress sensing chip which can detect the three-dimensional stress components separately is developed. The thermal stress occurs in the silicon chip almost linearly with temperature at the steady state. The stress path with temperature during heating is the same as that during cooling. On the other hand, during temperature cycling, stress hysterisis is observed, and the stress increases gradually with increasing cycles. The stress increase phenomenon occurs due to the viscoelastic phenomenon of the molding compounds. The stress increase-ratio is determined by the relationship between the cycle period and the stress relaxation time of the molding compounds at temperatures of interest.