Micro-strain effects, induced by thermal cycling in short fiber reinforced AlSi12CuMgNi piston alloy and AE42 magnesium alloy, are discussed. Above a critical temperature, the thermal strain curves deviates from linearity, leading to the appearance of a knee that is attributed to the beginning of matrix deformation caused by the internal thermal stresses. The thermal strain response is affected by the reinforcement, fiber orientation, thermal cycles and heat treatment. It is found that the thermal strain in the longitudinal direction is less than that in the transverse direction. The results show that any heat treatment to increase the matrix strength could lead to the disappearance of the knee.