HIROYUKI TODA, SEISHI NISHIDO, TOSHIKAZU AKAHORI, MITSUO NIINOMI, and TOSHIRO KOBAYASHI Hypoeutectic Al-Si alloys consist of primary ␣-Al and Al-Si eutectic phases and show typical elasticplastic fracture. To understand their fracture behavior, fracture processes were simulated using an elastic-plastic finite-element method. The validity of the J-integral-based criterion was verified and applied to the simulations. A complicated model was used to simulate the fracture in an idealized dendritic microstructure, and four simplified models were intended to more clearly understand the interaction between a crack and individual ␣ phases. Results show that the crack is attracted to the soft ␣ phase when passing by the ␣ phase, whereas it is repelled when the ␣ phase is close in front of or behind the crack tip. The presence of ␣ phase close in front of or behind the crack tip leads to an amplification of the driving force. However, the ␣ phase beside the tip reduces the driving force. Furthermore, the fracture behavior is mainly affected by the adjacent ␣ phase on one side around the crack tip, while the remote ␣ phase on the opposite side has an offsetting effect. The local stress-strain fields were examined to analyze the simulated behavior. The simulated crack-growth path in the dendritic model was compared and verified with the experimentally observed path.