A substantial approach was proposed to design polymer alloys with very low coefficient of linear thermal expansion (CLTE). The large reduction in CLTE is not based on an addition of low-thermal-expansion filler to suppress the bulk expansion, but on the fine control of the polymer alloy's micro-morphology so that the expansion is preferentially toward the thickness direction. In this study, rubber was used as an additive to tune the thermal expansion behavior of various plastics. Although the rubber has a high thermal expansion coefficient, it was found that, when the rubber domains are deformed into microlayers and co-continuous with the plastic matrix, the CLTE of the polymer alloy parallel to the microlayer directions could be reduced to a very low level. Various influencing factors, including rubber concentration, viscosity ratio, interfacial adhesion as well as the domain size were investigated. (C) 2004 Published by Elsevier Ltd.