The study assessed teachers’ perceived level of knowledge on preparation of lesson plans towards facilitating classroom instructions in secondary schools. Systems theory was used to guide the researcher in conducting the study. The target population of the study was fifty-one secondary schools found in Rombo District. Thirteen secondary schools were sampled using simple random sampling. In each school, seven teachers were sampled through stratified random sampling technique, three Ward education officers through simple random sampling technique and thirteen heads of schools participated in the study. The study used Convergent design under mixed methods approach. Questionnaires were used to collect data from teachers, interview guide to heads of schools and Ward educational officers. Document analysis guide was used to assess lesson plans prepared by teachers. To ensure content validity of instruments two experts from Mwenge Catholic University were consulted to review the instrument. Cronbach Alpha was used to assess reliability of the Likert scale in questionnaire, and for qualitative instruments estimation of reliability was ensured through member checking. Quantitative data were analysed using descriptive statistics and were presented in frequency tables, percentages and mean by the aid of SPSS version 27. Qualitative data were analysed by using content analysis by the aid of Computer software Nvivo 11 Plus and presented through narration and quotations. It was established that majority of teachers perceived to be very knowledgeable on preparation of lesson plans towards facilitating classroom instruction in secondary schools. Conclusion drawn from the study asserts that teachers perceived to be very knowledgeable on preparation of lesson plans although they had some problems in preparing some of the parts of lesson plans such as writing lesson competence. The study recommended that, the government and other owners of secondary schools should provide in-service training to their teachers yearly or after every two years so as to update their knowledge on lesson plan preparation.