An alloying element of 0-1.5 wt.% Si was added to an Fe-42%Ni system, and alloy strips were fabricated using a melt drag casting process. The effects of the Si and annealing treatments on the thermal expansion properties of Fe-42Ni alloy were investigated. The addition of Si enlarged the coexisting temperature region of the solid-liquid phase and reduced the melting point, which improved the formability of the alloy strip. An alloy containing 0.6 wt.% Si had a lower thermal expansion coefficient than any other alloy in the temperature range from 20 to 350 degrees C. The grain size increased with the rolling reduction ratio and annealing temperature, which caused an increase in magnetostriction and consequently a decrease in the thermal expansion coefficient of the strip. The alloy strip containing 1.5 wt.% Si had a higher thermal expansion coefficient than the alloy containing 0.6 wt.% Si because of grain refining caused by the precipitation of Ni3Fe.