Abstract: A stabilizer or dispersing agent for use in a suspension polymerization of a vinyl compound is provided which comprises a modified and partially hydrolyzed polyvinyl alcohol having an average polymerization degree of from 200 to 3,000 and an average hydrolysis degree of from 60 to 90 mol % and including anion hydrophilic groups at the side chains thereof, wherein said anion hydrophilic groups are selected from the group consisting of dicarboxyl groups and/or esters thereof and/or salts thereof, sulfuric esters and/or salts thereof and phosphoric esters and/or salts thereof, and the content of said anion hydrophilic groups is in the range of from 0.02 to 10 mol % per unit mol of the polyvinyl alcohol. A process for preparing a polymer of a vinyl compound is also provided wherein said vinyl compound is polymerized in a suspension thereof suspended by the use of the stabilizer or dispersing agent.