The present invention has its object to provide a liquid-form modified product of polyhedral polysiloxane which is excellent in moldability and transparency, and a composition produced using the modified product. In addition, the present invention can provide an easy-to-handle modified product and composition. The present invention provides a modified product of polyhedral polysiloxane which is obtainable by modifying a polyhedral polysiloxane compound (a) with a compound (b), and a composition containing the modified product. The polyhedral polysiloxane compound (a) has an alkenyl group and/or a hydrosilyl group, and the compound (b) has a hydrosilyl group and/or an alkenyl group each capable of hydrosilylation with the component (a).