The study investigated the influence of principals' workload on instructional supervision practices in public secondary schools in Lower Yatta Sub-County of Kitui County, Kenya. The study objectives focused on establishing areas that principals checked in instructional supervision, determining extent to which instructional supervision was perceived important and effective by teachers and rating of the extent to which principals work load influenced instructional supervisory practices. The study adopted descriptive survey design. The population consisted of 26 principals and 115 teachers in all the public secondary schools of Lower Yatta Sub-County. The entire population was involved in the study because the size was manageable. Questionnaires were used to collect data from principals and teachers. Quantitative data were analyzed using descriptive statistics and the results presented using frequency tables, pie charts, bar graphs and percentages to make meaningful conclusions. Qualitative data were analyzed through content analyses by organizing data into themes, patterns and sub-topics based on research objectives. The findings were that the areas covered by the principals in instructional supervision were: schemes of work, lesson plans, record of work covered, pupils' lesson notes and class attendance. The study also established that instructional supervision was considered significant and personally benefited teachers in their professional in respect to syllabus coverage, professional development, and improvement of the instructional process. Finally the study established that high administrative work load adversely affected effectiveness of principals in their supervision.