This study focuses on the grammatical errors in the essays written by JSS1 pupils in five junior secondary schools in Bo city. It aims at identifying the errors occurring in the use English grammar by pupils, classifying the errors, determining their frequency levels and their sources. The descriptive survey design was used in this study, where out of a population of 853 pupils, 250 pupils were used a sample for this study. Each of the pupils was asked to write an essay, and the scripts were marked to identify all grammatical errors made by the pupils. From this study, it was discovered that out of 250 scripts, 32% of the pupils were not able to communicate their thoughts meaningfully. Out of the comprehensible scripts the following errors were identified and their frequencies: wrong use of tense 29.54%, errors in spelling 24.67%, errors in the choice of words 13.13%, wrong use of prepositions 7.79%, verb addition errors 5.84%, preposition addition errors 4.87%, pronoun addition, subject-verb agreement errors, wrong use of pronouns, plural and verb omission errors all received 1.62% each. The study further revealed that a good number of errors discovered from students’ writings were intralingual errors.