Risk is the outcome of an event which may be predicted on the basis of statistical probability. Construction projects have inherent risks associated with it. The main aim of this paper is to identify the critical risk factors and to propose a model to find the probability of cost deviation in medium sized construction projects. A total of 46 risk factors were shortlisted based on pilot study and experts' opinion. A questionnaire survey was conducted among 223 various construction professionals each representing one project i.e. from 223 different projects. Based on factor analysis, 46 risk factors which influence the cost deviation in construction projects were classified under nine key risk groups viz. project scope and evaluation risk, work environment risk, documentation and legal framework risk, construction and operational risk, resource productivity risk, knowledge sharing risk, site supervision risk, financial viability risk and lean construction risk. Logistic regression analysis was also carried out to develop a model to find the probability of cost deviation in construction projects. It is concluded that the risk groups such as project scope and evaluation risk, site supervision risk, knowledge sharing risk and lean construction risk are having higher influence in the cost deviation in medium sized construction projects. By setting the effective baseline of the project like estimation of original project cost and detailed project report, cost deviations in medium size construction projects can be eliminated. Detailed recommendations are also provided.