The objective of this study was to assess the causes of conflicts and the effects of conflict in the secondary school of Arada Sub-City. The study was conducted through descriptive survey research design and both qualitative and quantitative methods were employed to analyze data. The study was carried out in six secondary schools of Arada Sub-City of Addis Ababa City Administration which were selected by availability sampling techniques. Data were collected by the researcher from 6Woreda education office head, 22 principals, all WEOH and principals were included in the sample and 162 teachers were selected through simple random sampling, using questionnaires and interview techniques. Data were analyzed using frequency count, percentages and reported in tables. The findings were that the cause of conflict was lateness to school and absenteeism from work, poor communication and inadequate teaching and learning materials as reported by the respondents. It was also found that the effects of conflicts decrease co-operative learning at school and reduce teachersâ€˜ motivation. Based on the analysis of the data, the following conclusions were made: the cause of conflict between school principals and teachers were lateness to school and absenteeism from work, and poor communication, whereas the effects of conflicts decrease co-operative learning at school and reduce teachersâ€˜ motivation. As result, the recommendations forwarded were the school principals should try to find out the cause of conflicts in secondary schools and involve all the education stakeholders in the school in order to enhance discipline for effective conflict management strategies to be put in place. Moreover, awareness should be raised for principals and teachers related to effective conflict resolution at schools so that each one can be involved in the process.