The importance of accounting information system in any organisation, especially the small and medium scale enterprises cannot be over emphasised. The accounting information systems employed by SMEs are very crucial to their performances. Due to lack of accounting information, SMEs however, make wrong decisions. In light of this problem, this study sought to determine the effect of accounting system on the performances of SMEs in Nigeria. The population of this study consists of the Small and Medium scale Enterprises (SMEs) in Festac – Town, Lagos. Data were extracted from 154 questionnaires administered with 80% retrieval success. The hypotheses were formulated and tested using regression analysis at 5 per cent level of significance (0.05).The data were analysed and interpreted using both descriptive and inferential statistics. The study found accounting information system having a significant positive effect on SMEs performance. In conclusion, accounting information systems employed by the managers/owners of SMEs were found to have contributed positively to their decisions and performances. We therefore recommended that users of accounting information should take cognizance of the quality of accounting information systems provided so as to aid their performance.