The notch fracture toughness of Fe₇₅Mo₅P₁₀C_8.3B_1.7 monolithic bulk metallic glass (BMG) and Fe₇₇Mo₅P₉C_7.5B_1.5 and Fe₇₉Mo₅P₈C_6.7B_1.3 BMG matrix composites with α-Fe as reinforcing phase, fabricated by suction mould casting, were evaluated. It was found that the monolithic BMG has a toughness of 27 MPam^1/2, while the Fe₇₇Mo₅P₉C_7.5B_1.5 BMG composite reinforced by single α-Fe dendrite phase exhibits a higher toughness of nearly 40 MPam^1/2. However, for the Fe₇₉Mo₅P₈C_6.7B_1.3 alloy with more dendrites, the toughness decreased up to 25 MPam^1/2. Microstructure investigation reveals that the simultaneous formation of Fe-Mo-P hard brittle phase apart from α-Fe dendrites in the Fe₇₉Mo₅P₈C_6.7B_1.3 alloy is the reason for the degradation of the fracture toughness.