The study assessed the planning practices of principals for effective school administration in Secondary Schools in Anambra State, Nigeria. The study adopted a descriptive survey research design which was guided by three research questions and one null hypothesis, while all the 257 principals of public secondary schools in Anambra State were used as sample because of the size. A questionnaire structured by the researcher, titled Assessment of Principals’ Planning Practices Questionnaire (APPPQ) was the instrument used for data collection. The instrument was validated by three experts, two experts from Educational Management Unit and one expert from measurement and evaluation unit all in Chukwuemeka Odumegwu Ojukwu University, Igbariam. The reliability co-efficient of the instrument was calculated to be 0.73 using Cronbach Alpha formular. Research questions were answered with mean and standard deviation while t-test statistics was employed to test the null hypothesis. The findings revealed that ineffective planning causes poor teaching and hazy preparation of budget. Inadequate fund to procure materials needed for planning and principals’ not involving the concerned staff in planning are also causes of poor planning. Based on the findings, the researcher recommended that; government should provide periodic in-service training on planning for principals and principals should practice good communication skills.