Lesson planning is considered to be an important and efficient tool for effective teaching and learning process. Preparations of effective lesson plan requires teachers to be competent in the pedagogical content knowledge (PCK). This study investigated how the mathematics teachers' PCK impact their competences on designing effective lesson plans. Twelve in-service mathematics teachers from public secondary schools in Unguja-Island (Zanzibar) of Tanzania were involved. The data were collected using the reviewing of the teachers' mathematics lesson plan documents and teachers' interview. The data were analyzed in both quantitative and qualitative mode based on the lesson plan framework guideline (LPFG) and the criteria for better planning of the sections jointly with PCK abilities indicators. The percentage average of occurrence of the sections in the teachers' designed lesson plan (DLP) and the percentage of occurrence of different category within the sections of the DLP in relation to the criteria were determined. Also, the challenges faced by the teachers in their DLP were identified. The result of the analysis revealed that the PCK competences of mathematics teachers are not good enough to impact their DLP, as some of their designing lesson plan sections were occurred inconsistently. Also, the implementation of PCK competences for effective mathematics teaching found to be at developing stage, as some criteria related to it were not clearly observed in their DLP. The study recommends the demand of in-service training for mathematics teachers on the implementation of teachers' competences particularly PCK in the lesson plan designing for effective classroom practices.