The research sought to investigate prepositional errors on English usage. There has been a big outcry in the field of English language learning about the falling levels of proficiency in English among students, particularly in upper primary level. This problem was attributed to inadequate mastery of linguistic skills and incomplete acquisition of English language. Therefore, the current paper investigated preposition errors as manifested in written compositions of upper primary learners. Prepositional rules in English language are inconsistent; that is certain prepositions can be applied in one form, but not in another. More so, prepositions are polysemous. Thus, learners often become frustrated when they have to determine prepositional meanings and when to use them appropriately. The research was guided by the following objectives; identifying prepositional related errors in learners' composition and describing the errors. The study applied Corder’s(1974, 1976) Error Analysis Theory. The study adopted a descriptive research design in order to vividly describe various errors encountered in the usage of English prepositions. The target population was 52 primary schools,168 teachers of English, and 408 pupils from Public Primary Schools in Kieni-East Sub County in Nyeri County. Data was collected using interview guides for the teachers and composition test for pupils. The study showed four main prepositional errors; errors of omission, addition, substitution and misordering. The study concludes that prepositional errors are more of lexical errors since they affect the meaning of the entire sentence. Thus, the study recommends that learners should be taught rules of prepositions systematically and that a revision of course books should provide specific rules that may lead to proper acquisition of prepositions.