PURPOSE: To obtain an modified alkyd resin free from gelation, capable of affording slightly yellow, transparent varnish, by incorporating an alkyd resin with a cyclopentadiene resin prepared by thermal copolymerization of a specific monomer mixture.

CONSTITUTION: During or after completion of a reaction between (A) a polyhydric alochol such as glycerin, (B) a polybasic acid such as phthalic anhydride, and (C) an oil such as tung oil, (D) pref. 5W40wt%, based on the final product, of a cyclopentadiene resin prepared by thermal copolymerization of a monomer mixture consisting of (a) 90W60 parts by wt. of a cyclopentadiene monomer and (b) 10W40 parts by wt. of a 5C linear-conjugated diene is added to the reaction system. The resulting sustem is heat-treated to obtain the objective modified alkyd resin.

EFFECT: Capable of giving a coating film of high gloss, hardness, water and alkali resistance, and good drying characteristics.