This study investigated the effects of senior secondary school students' use of demo kits on their achievement in biology in Omu-Aran, Nigeria. The study utilized a quasi-experimental design and followed the non-randomized, non-equivalent pre-test and post-test control group of 2x2x3 factorial matrix. Data was obtained from four intact classes in four purposively selected secondary schools in Omu-Aran, Kwara State, Nigeria. The research instruments - Biology Achievement Test (BAT) and demo kits were subjected to item analysis and validated by three science education experts from a university. The reliability of the BAT was determined using test re-test method while Cronbach alpha was used to analyze the resulting data. Descriptive statistics, standard deviation and mean gain scores were used to answer the three research questions while Analysis of Covariance (ANCOVA) was used to test the corresponding null hypotheses. Findings from the study revealed that there was a significant difference in the achievement of students who learned biology using demo kit and those who learned it without using demo kit since F(1, 170)= 112.48, p = 0.000; there was, however, no significant difference in the achievement of male and female students because F(1, 94)= 2.134, p = 0.147; significant difference was observed in the achievement of students in the various score levels in favour of the high scorers. Based on the findings, it was recommended, among others, that biology teachers consider exposing their students to the use of demo kits to enhance meaningful learning and retention of information by the students.