This study investigated the impact of assessment methods used in internal evaluation on students’ achievement in English grammar in secondary schools in Kenya. This study was anchored on Vygotsky’s Sociocultural Theory. A pragmatic paradigm was adopted using exploratory sequential mixed methods. A quasi-experimental, pre-test, and post-test design was adopted. The target population comprised of all the form two students and teachers of English in Teso North Sub-County. Stratified and simple random sampling was used to select 10 schools. Simple random sampling was used to select 509 students. Purposive and simple random were used to select 10 teachers. Data was generated using an interview schedule, observation schedule, questionnaire, and English Grammar Achievement Tests. Quantitative data was analyzed using descriptive and inferential statistics. Qualitative data was analyzed using the narrative analysis technique and presented through narration and direct quotes. Findings revealed that grammar was predominantly assessed through gap-filling as well as question and answer method. Assessing grammar through cloze test and composition writing had a more positive influence (M = 16.02; SD = 3.51) than where it was not used (M = 14.12; SD = 3.33). Pearson Correlation revealed that assessment had a statistically significant relationship of r = .643, n = 509, p = .006 with students’ achievement in English grammar. The study concluded that cloze tests and composition writing are effective methods of assessing English grammar. It was recommended that the Ministry of Education should in-service teachers on the importance of assessing English grammar through these methods. This study recommends that teachers should assess English grammar using cloze tests and composition writing.<p> </p><p><strong> Article visualizations:</strong></p><p><img src="/-counters-/soc/0758/a.php" alt="Hit counter" /></p>