Cast plates of A356 aluminum alloy with different thickness were fabricated by rheocasting and squeeze casting. Tear tests were performed on the as-cast and heat-treated products (T5 and T6), and effects of solidified structure and heat treatment on unit crack propagation energy (UEp) were examined. Increased cooling rate (corresponding to a decrease in plate thickness) resulted in refined solidified structure and enlarged UEp values for as-cast samples. For both rheocast and squeeze-cast samples, both spheroidized eutectic Si particles and age-hardened α-Al matrix by T6 treatment were effective for increasing UEp. UEp of the squeeze-cast sample was higher than that of the rheocast sample. Observation of crack growth path and fracture surface revealed that the tear toughness of the present cast alloy was controlled by distribution of eutectic solidified region in the cast structure, which provided a preferential crack growth path. The discontinuous distribution for the squeeze-cast sample is considered to be beneficial for increasing crack growth resistance rather than the continuous arrangement of the network-like eutectic region for the rheocast sample. Relatively small UEp was obtained for the T6 treated rheocast sample collected from the 6 mm thickness plate. This is attributable to the volume fraction of the eutectic solidified region in the sample being larger than others under the present experimental conditions.