A process for preparing a flame-retardant phosphorus-containing unsaturated polyester resin blend, comprising the following steps:(a) reacting(1) a phosphorus-containing monomer having the formula: ##STR1## wherein R.sub.1, R.sub.2 and R.sub.3 are respectively H, C.sub.1 -C.sub.4 alkyl, phenyl, alkaryl or aralkyl, with(2) a saturated dicarboxylic acid, or a saturated dicarboxylic acid anhydride,(3) a double bond-containing unsaturated dicarboxylic acid or a double bond-containing unsaturated dicarboxylic acid anhydride, and(4) a diolin the presence of a metal catalyst combined with an acid to form an unsaturated polyester oligomer having the formula: ##STR2## wherein R.sub.5 and R.sub.6 are respectively C.sub.1 -C.sub.4 alkylene, alkarylene, phenylene or aralkylene,R.sub.4 is a vinyl-containing group,x is an integer of 1-10,y is an integer of 1-10,z is an integer of 1-10, and(b) blending said unsaturated polyester oligomer with a vinyl-containing monomer.