PURPOSE: To provide the subject composition comprising a specific allyl ester, a specific curing agent and an unsaturated polyester resin in a specified ratio, not generating cracks, low in contraction rate, excellent in hardness, mechanical strength, water resistance and transparency, and useful for molded products such as plastic bathtubs, etc.

CONSTITUTION: This composition comprises (A)2-60wt.% of an allyl ester oligomer having an unsaturation degree of 2-45 represented by an iodine value and a number-average mol.wt. of 1000-50000 converted into polystyrene and measured by GPC, (B) 40-98wt.% of an unsaturated polyester resin, and (C) a tertiary alkylperoxy 2-ethylhexanoate of the formula (R is methyl, ethyl, tert-butyl) such as tertiary amylperoxy 2-ethylhexanoate in an amount of 0.1-10 pts.wt. per 100 pts.wt. of the components A+B. The composition is preferably cured at 40-120°C.