This study examined the effects that computer-assisted instruction had on students' attitudes toward a mathematics lesson and toward learning mathematics with computer-assisted instruction. The computer software we used was based on the ASSURE Instructional Systems Design and the ARCS Model of Motivation, and the software was designed to teach fractions to fourth-grade students. The skill levels of these students were gauged before and after receiving the computer-assisted instruction. We structured our experimental design to use one group for both pre- and post-tests, which is considered to be one of the weak experimental designs. We conducted our research with 28 students studying in Balikesir, Turkey, for a period of six weeks, using the specifically developed teaching material. We gathered our research data by applying an attitude scale to our mathematics lesson and to computer-assisted instruction. We also applied the Academic Achievement Test for Fractions Unit in Mathematics, a test we developed for our research. We analyzed our gathered data with the Wilcoxon Signed-Rank Test (n<30) by using the statistical software SPSS 15. The conducted analyses showed that the activities of the developed instructional material had positively affected the attitude of the students toward computer-assisted instruction (z = -2.807, p< 0.05) and increased their academic success (z = -4.623, p<0.05). Although the attitude scale toward our mathematics lesson indicated an increase in their scores, this increase was not found statistically significant (z = -2.807, p>0.05). Based on the research results, we believe that similar materials can also be used for instructing other topics of mathematics, and similar computer-assisted activities can be developed for other courses.