For over twenty years now, there has been persistent outcry from the general public, parents and teachers that the performance in mathematics is low in primary schools. Language of learning and teaching (LOLT) mathematics has been pointed out as one of the factors that affect learners understanding of mathematics especially in classes where English as a second language is used. Therefore this study aimed at finding out the effect of language of instruction in learners understanding of mathematics especially those involving word problems in standard 5. The study used qualitative approach. The sample consisted of one hundred and thirteen learners and two mathematics teachers selected from two primary schools. Four data collection techniques were used. The data collected were analysed using qualitative data analysis techniques. The study later found out that English supplemented with Chitumbuka was used to a larger extent as a language of learning and teaching in standard 5 during mathematics lessons. The findings of the study also revealed that using English as a language of instruction when teaching and learning mathematics in standard 5 resulted into a number of common language problems, misconception and errors. Hence the study established a number of strategies that both teachers and learners used or would want to use in order to overcoming problems. From these findings it was concluded that the language of learning and teaching mathematics in primary school has an effect on learners understanding of mathematics especially those involving use of word problems.