A Dissertation Submitted to Mzumbe University Dar es Salaam Campus College in Partial Fulfillment of the Requirements for the Award of the Degree of Master of Science in Accounting and Finance (MSc-A&F) of Mzumbe.This Research was primarily concerned with exploring IAS in domestic airline companies the case of AAS and NA. The motive was to provide comparative empirical support for policy formulation, implementation and rectification with regard to the effectiveness and challenges of IAS whilst contributing to the limited body of knowledge on auditing in airline business in Tanzania perspective and Africa in general. The parties involved in the study included: Management and staff of AAS and NA. The study employed questionnaires, interviews, content-analysis and the use of various documents as the major instruments for gathering data. Data were analyzed using Microsoft Word and Excel Applications and SPSS package version 11.5, qualitative data were categorically organized. The findings from the analysis of data discovered that Internal audits are designed to evaluate the effectiveness of an operation's internal controls by first gathering information about how a unit operates, identifying points at which errors or inefficiencies are possible, and identifying system controls designed to prevent or detect such occurrences. Findings revealed that IAS at AAS was more effective as compared to NA as reflected in how indicators are adhered to. Several issues that need to be addressed are commented. Recommendations are thereof given on how to ensure effective operation, management and performance of the IAS in domestic airline companies’ in Tanzania. An internal auditing system must have the commitment of senior management. Without their approval, support, and encouragement, the internal audit process is doomed for failure and worse-time and money wasted.