Many studies have been done on human-computer interaction (HCI), but there is a dearth of studies on the mandatory actual system use in Nigerian university libraries. The relationship between sources of computer self-efficacy and outcome expectancy on actual system use was assessed, based on social cognitive theory, by surveying mandatory use of KOHA version 3.0.1 integrated library management software. The questionnaire survey was conducted on 61 library personnel in 5 private Nigerian universities. Data were analyzed by multiple regression analysis and Pearson product moment correlation. The structural model accounted for 48% of variance in actual system use, 80% in computer self-efficacy, and 19% in outcome expectancy. This study suggests that usability, supportive management, and computer self-efficacy are important determinants of actual system use of KOHA software. Self-efficacy had influence on outcome expectancy. The implication of applying the social cognitive theory to actual system use of KOHA software by library personnel is discussed.