The aim of this paper is studying the effect of timely disclosure on the relation between voluntary disclosure and cost of equity capital. In this regard, a Pourheidary's disclosure checklist, Gordon’s growth model and a dummy variable based on Ashton et al definition were respectively used for measuring companies’ disclosure score, their cost of equity capital and their timely disclosure. Also, the number of one hundred eleven companies listed on Tehran Securities Exchange during 2010 to 2014 (totally 560 firm-year observations) were selected as the sample of research using systematic screening method. Furthermore, required data related to these observations were collected from ‘Codal’ database and ‘Rahavard Novin’ software. The results of hypotheses testing applied with regression analysis showed a significant negative relation between voluntary disclosure and cost of equity capital. Also, tests implicate a significantly negative association between timely disclosure and cost of equity capital. In addition, the tests demonstrated that interaction effect of voluntary and timely disclosure on cost of equity capital is negative and significant. In the other words, the reducing effect of voluntary disclosure on cost of equity capital of companies that timely disclose their information is more than companies that untimely disclose their information.