Trainers are most important in National Polytechnics, in the technical vocational education and training (TVET) system as they are fulcrums upon which wheels of the education rotate in delivering competence needed to convert global labour-oriented economy to a knowledge and innovation-based economy. This paper was guided by four trainer pedagogical competence objectives, which included assessing training techniques, training tools, instructional media, and communication skills as they affected the diploma courses of the Science, engineering and technological (SET) courses of students’ academic performance in the National Polytechnics in western Kenya. A descriptive survey design was used to collect both quantitative and qualitative data from the three national polytechnics in western Kenya. A finite sample of 112 trainers' fraternity respondents were chosen using stratified selection and simple random sampling methods from a population of 650 trainers. Utilizing questionnaires and interview schedules, data was gathered. Data collecting tools were tested for reliability and were subjected to test-retest procedures, which gave 0.782, and was within the acceptable range as suggested by Mugenda & Mugenda. The primary questionnaire's items were arranged in accordance with the specific goals of the study. Descriptive statistics and Pearson correlations were used to analyse the data. The study found that the trainer's pedagogical competencies of instructional media had the strongest statistically significant relationship with students' academic performance in national Polytechnics (r=.780, p.05), followed by communication skills (r=.618, p.05), and training methods and use of training tools (r=.599, p.05) respectively.