To provide an inexpensive grafted modified terpene resin which excellently improves polymer materials, etc., and an olefin resin composition having an improved paintability using the modified terpene resin.

This modified terpene resin is obtained by allowing a terpene resin to react with an olefinically unsaturated monocarboxylic acid and/or dicarboxylic acid or anhydrides thereof in the presence of a radical initiator in the absence or presence of a solvent at from 0 to 20°C. An olefin resin composition is obtained by blending 100 pts.wt. olefin resin with from 0.1 to 40.0 pts.wt. modified terpene resin.