The study investigated the differential achievement of students in Yor6bq language based on teachers’ teaching experience and professional qualifications in Ilesa Metropolis, Osun State, Nigeria. Teacher’s Competency Theory was used to back up the study. The research design adopted was the mixed methods (Quantitative and Qualitative). Two hundred and twenty (220) senior secondary school students and five Yor6bq language teachers participated in the study. The instruments used for data collection were Teachers’ Qualification and Experience Scale (TQES), Yor6bq Language Achievement Test (YLAT) and In-depth Interview Guide (IIG). Data collected were analysed using t-test at 0.05 level of significance. Findings revealed that there are significant differences in the performance of senior secondary school students taught by professionally qualified and non-professionally qualified and by experienced and inexperienced Yoruba language teachers. This implies that professional qualification and teaching experience are key requirements for effective teaching of Yoruba language in the senior secondary school. Based on the findings, it is recommended that the Yoruba language teachers should attend conferences, seminars, workshops, pre- and in-service training programmes for their growth. They must also continually develop themselves through higher degree programmes.