This article is based on a study whose main objective was to determine the effects of mathematical vocabulary instruction on students' achievement in Mathematics in Secondary schools in Murang'a County, Kenya. The study was a non-equivalent control group pretest-posttest quasi-experimental design and was conducted in the two purposively selected secondary schools in Kahuro District in Murang'a County, Kenya. Both the experimental and the control groups consisted of fifty four (54) students from each school. The experimental groups were exposed to mathematical vocabulary instruction using the Graphical Organizer based on the Frayer Model with ICT integration instructional approach for ten (10) weeks. The control group was taught mathematical vocabulary by the definition-only method for the same period. Data were analysed using one-way ANOVA, independent t-test and paired t-test. The findings indicated that there is a statistically significant mean difference in the students' performance in Mathematics between those taught Mathematics vocabulary using the Frayer Model with Technology and those taught Mathematics using the definition-only method.