The invention provides a lactone-modified organopolysiloxane compound represented by the formula ##STR1## wherein R.sub.1, R.sub.2, R.sub.3 and R.sub.4 are the same or different and each represents a methyl group, an ethyl group, a n-propyl group, an isopropyl group or a group represented by the formula ##STR2## wherein R.sub.5 is a methyl group, an ethyl group, a n-propyl group or an isopropyl group, X is a saturated divalent hydrocarbon group having 2 to 14 carbon atoms, and m is an integer of 1 to 3, provided that at least one of R.sub.1, R.sub.2, R.sub.3 and R.sub.4 is a group represented by the formula ##STR3## wherein R.sub.5, X and m are as defined above, and n is an integer of 1 to 100. The polysiloxane compound of the invention is excellent in compatibility with various organic resins.