Timeliness of financial information, as a primary characteristic of value relevance of accounting information, is considered a necessary determinant of the quality of financial reporting investors need to take rational investment decisions. Extend literature have paid attention to factors affecting timeliness of financial reporting. Joint audit as one of these factors, However, mixed results had been found concerning the potential impact of joint audit on timeliness of financial reporting. The mixed results could be related to other moderating variables; one of these variables is, as referred to in prior research, size of audit office. The purpose of this study is to investigate the impact of size of audit office on the relationship between joint audit and annual report lagging (as a proxy for timeliness of financial reporting). Our study uses a sample from publicly Egyptian companies representing different sectors consisting of 960 observations during 2005 to 2020. Using pooled regression models, the research found a significant impact of the joint audit (especially if one of the auditor is of the Big 4) on the timeliness of financial reporting.