major yardstick in assessing a government lies in the quality and outcomes of summative examinations during the regime – especially in Senior School Certificate Examinations being conducted by West African Examinations Council (of which Nigeria is a member) and National Examinations Council (which domiciles in Nigeria). This study examined the power of professionalism (qualification, specialisation, years of experience and in-service training) on English language teachers’ effectiveness in Southwest, Nigeria. Through descriptive research design of survey type, a multistage sampling technique was employed to select the two hundred and ninety-four (294) English language teachers as respondents to an adapted English Language Teachers’ Effectiveness Self-Evaluation Scale (ETESES) containing three sections on Demography, Teachers’ Professionalism Checklist (TPC) and their perception of certain classroom practices. Data generated were analysed with descriptive and inferential statistics: Mean and Standard deviation were used for research questions; while Pearson Product Moment Correlation, Analysis of Variance, T-test, Multiple Regression and Duncams Multiple Range Test were used for testing hypotheses at 0.05 level of significance. Teachers’ Professionalism yielded certain rates, as revealed in years of experience, educational qualifications, subject specialisations and professional development (β = .059, 0.337 and 0.254; p < 0.05), on their overall effectiveness. The study, therefore, concluded that professionalism has positive effect on English language teachers’ overall effectiveness. Consequently, the study, recommended among others, that professionalism should be considered at the point of teachers’ appointment. Also, the yardsticks for assessment of teachers for appointment and promotion should involve elements of reflective thinking on preparation and classroom practices.