Short fiber reinforced AlSi12CuMgNi composites have potential applications as engine components in automobile industry. In these engine components, the dimensional stability is of great concern. Thermal cycling experiments can approximately simulate the real working conditions of the materials and give an evaluation of the dimensional changes during their service in the changing temperature environments. Due to the fact that in metal matrix composites the thermal strain is dependent on the phase transformation, the matrix plastic yielding and the physical damage of reinforcement, analysis of thermal strain curves could allow not only insight into their thermal expansion behavior but also into the phase transformation and possible matrix plastic deformation behavior caused by large internal thermal stresses. Compared with the thermal strain curve, the instantaneous CTE curve can clearly supply more information because it is the differential of thermal strain over temperature. It is useful to differentiate whether the appearance of a knee on the thermal strain curve is caused by the phase transformation and/or by the matrix plastic deformation, especially in metal matrix composites with age hardenable matrix. In the present paper, the aging behaviors in both the unreinforced and ceramic reinforced AlSi12CuMgNi piston alloys are investigated and discussed by analyzing the instantaneous CTE curve, DTA curve and hardness tests. The matrix plastic deformation caused by thermal stresses is discussed based on the observation of CTE divergence in the longitudinal and transverse directions. The effects of both the SiO₂ content and subsequent heat treatment on the critical temperature, above which the CTE divergence occurs, are discussed. Finally, the effects of precipitation and matrix plastic deformation on the residual strain after thermal cycling are also preliminary evaluated.