T his study seeks to investigate the effects of the frequency of lexical errors on the assessment of compositions written by the primary school learners, and factors that cause lexical errors in their writing in English through email communication. The data, 182 compositions, were gathered from 51 sixth grade learners during the second semester of 2015. The results revealed that the students had the greatest problem with formal mis-selection of words in lexical error category. The results showed that the type of writing prompt seemed to have an effect on the frequency of occurrence of lexical errors. Participants made more mistakes on interlingual/transfer errors than on intralingual/developmental errors. Learners also had intralingual errors due to the overgeneralizations and partial exposure to the target language. In addition, learners also included internet linguistic features in their writing. A low negative correlation was found between the holistic scores and the frequency of lexical errors. The results also provided evidence that as the frequency of lexical errors decreases the quality of the compositions relatively seems to increase.  Some implications are drawn this study. Teachers can include these errors in the teaching, and they should provide a context where fluent and accurate language use should be modeled to learners.