The present invention relates to pharmaceutical compositions comprising an anthelmintic agent. Specifically, the invention relates to a pharmaceutical composition comprising praziquantel or R- praziquantel in nanosized particles and one or more pharmaceutically acceptable excipients. The invention also relates to a process of preparing a pharmaceutical composition comprising praziquantel or R- praziquantel in nanosized particles and one or more pharmaceutically acceptable excipients; and its use in the treatment of anthelmintic infections.