Abstract : Construction industry has boomed recently in Bahrain. However, delays issues in construction are significant and that impact projects delivery within planned timeframe, cost, and quality standards. From here this research has a vitality of investigating and analysing delay factors which affect building construction projects in Bahrain specially in private sector. Fifty (50) delay factors and their background were stated through a comprehensive analysis of previous related literatures. This research followed non- random sampling method as the questionnaire was distributed to the main project parties including contractors, consultants, and clients/owners to provide their opinions regarding those delay factors by online and face to face. Population size of this research was N=5902 and total of 134 responses was collected. Factors of delays were classified through this study into eight categories: materials-related, equipment-related, manpower-related, project management-related, consultant-related, client/owner- related, governmental-related and external-related factors. Delay factors of this study were analysed and ranked by using Kruskal Wallis/Mann-Whitney and Relative Importance Index (RII) methodology. Kruskal Wallis/Mann-Whitney and Relative Importance Index (RII) for both frequency and severity of delay factors were applied to determine means and key delay factors influencing building construction projects. Based on overall results, key influencer factors are "continuous changes order by owner", "poor site management & Ineffective control system", "unrealistic cost estimation (underpriced) & cash flow", "lack of skilled manpower", "owner interference in construction operations", "un-realistic contract duration", "decision making sluggish by owner", "poor communication skills between contractor PM & project parties", "claims between client & the main contractors" and "inaccurate project planning & scheduling by contractor". Thus, respondents agreed that contractors and owners are the most responsible parties for construction delays. Although, this research was conducted on Bahrain building construction projects, but it can be applied to other countries and further studies.