The paper examines the incidence of theft and mutilation of library materials, and gives users’ assessment of library security and factors that influence theft and mutilation among library users in Nigerian university libraries. Questionnaire method was used to obtain data. Of the users surveyed, 62.63 percent admitted that stealing and mutilation were common phenomena among library users. Some reasons given for this are: limited copies of library books, fear that one may not find the materials on the shelf and negligence of library security men. Suggestions were made on how to deal with culprits caught. The paper concludes by offering some useful suggestions on the way out of these problems.