Instructional supervision is key to effective teaching. Whereas emphasis has been put on external instructional supervision, less attention has been given to Internal Instructional Supervision (IIS), probably because its influence on teaching effectiveness is yet to be established. Kakamega County is the second largest County in terms of population yet achievement in English is low at a mean score of 5.28 in Kenya Certificate of Secondary Examination (2007-2012). The County’s Panel of Standards Assessment report, 2010 and 2011 pegged this to weak IIS structures. The purpose of this study was to establish influence of sensitization of teachers of English towards IIS on their teaching effectiveness. A conceptual framework constituting of sensitization of teachers of English towards internal instructional supervision as the independent variables and teaching effectiveness as the dependent variable was used. The study used ex-post –facto, correlation and descriptive survey. Population was 13 Quality Assurance and Standards Officers (QUASO), 247 principals, 247 HODs and 494 English Teachers (ET). It sampled 13 QUASO, 74 principals, 74 HODs and 215 teachers purposively. Questionnaires, interview schedules and document analysis guide were used. To establish validity, the instruments were given to experts in language pedagogy. A pilot study was conducted using 10% of the population; hence, 24 principals, 24 HODs and 50 teachers were used to establish reliability. Quantitative data was analysed using frequencies, percentages and Pearson’s correlation coefficient and regression. Qualitative data was transcribed, categorized and reported in themes relevant to the study. Findings revealed that teachersof English had been greatly sensitized on professional documents but least sensitized on purpose of IIS and they were rarely sensitized on matters of IIS through in-service. The study concludes that sensitization of ET towards IIS is not a significant factor of predicting teaching effectiveness and only 10.4 % of teaching effectiveness can be predicted by sensitization of ET towards IIS. The study thus recommends that stakeholders of IIS should sensitize teachers of English on the purpose of IIS, frequency of conducting IIS, teacher preparedness towards IIS and exposure through in-service since these are positively significant to teaching effectiveness.