The purpose of this study was to examine the contribution of instructional supervision in improving teachers’ teaching practices in government general secondary schools of Assosa Zone and Assosa Administrative City. Mixed research method particularly concurrent mixed method was used to carry out the study. The target populations were all teachers and internal supervisors in the government secondary schools in the study area. The sample consisted of 145 respondents consisting of 92 teachers 33 Department heads, 7 unit leaders, 5 supervisors and 2 vice principals were selected by comprehensive sampling method. 6 principal were selected by purposive sampling method from 6 government secondary schools. Questionnaire was the main data gathering instrument for this study. The researcher pilots the instruments to check its reliability by administering to 30 teachers and the result of the pilot testing was statistically computed by the SPSS computer program. The reliability coefficient of the instrument statistically calculated and found the Coefficient of Alpha (∝) b/n 0.763 and 0.888. An interview also conducted to enrich the quantitative data. Descriptive statistics such as frequencies, percentages mean, standard deviation and independent t-test were used to present the findings. The following were the findings of the study; the internal supervision activities under the study area were at good practice. The contribution of active learning was at higher level, Classroom management and student assessments were at medium level, action research was low as compared with others. The major challenges that general secondary schools internal supervisors come across to implement internal supervision were; they were workload with other tasks that means they taught the same credit like other teachers and absence of financial incentive. The following recommendations were drawn from the findings; internal supervisors should be regularly support the instructional activities. Training, monthly meeting, and daily one to five discussions should be emphasis on improving teachers’ teaching practices. It could be recommended that, the concerned bodies together with schools principals should reinforce the internal supervisors in supporting teachers in preparing valid assessment methods in general secondary schools. Advising to use table of specification, looking at objective of the content and considering the Minimum Learning Competences (MLC) of the student is very important in order to able to prepare valid test. The concerned bodies should support teachers to do action research, how to conduct and implement action research training should be given for teachers. Principals and vice principals should reduce the internal supervisors workloads and make them concentrate on internal supervision. The schools should be search some income generation by themselves or by asking education office to consider this during the budgeting to provide incentives for internal supervisors to motivate them for effective implementation of internal supervision in general secondary schools. Finally, the study come to end by recommending that Regional Education Bureau, Zone and Administrative City Education Office experts with local Education Colleges and University experts, who work on the area of supervision and support services to provide adequate training for internal supervisors to encourage teachers to implement CPD effectively and conduct action research to improve teachers’ teaching practices.