The purpose of this study was to investigate the effect of time of the day and duration of a test on secondary school students’ mathematic achievement. A 3 x 4 factorial balanced design was applied. A 50-item multiple choice mathematics achievement test and two equivalent forms of it were administered on a sample of 488 students, randomly assigned to 12 treatment combinations of time of the day (morning, afternoon and evening) and duration (45 min, 60 mins., 75mins, 90mins). The parallel form reliabilities were and where form A was administered in the morning, form B in the afternoon and form C in the evening. The students mathematics achievement scores were recorded as number right. The resulting data were analyzed using two-way ANOVA while F-ratio and LSD tests were used to test for significance of effect. The results showed that test duration, time of the day and their interaction effects on students’ mathematics achievement, were significant, with those who took the test for 90 mins and in the afternoon, obtaining the highest mean score (), while those who took the test for 45mins and in the evening obtaining the least mean mathematics achievement score (). The implications of those results to test designers and administrators are discussed.