Abstract: With therecent trend towards deregulating power systems around the world, assessment of Available Transfer Capability (ATC) and Congestion Management have become key requirements for the smooth running of power markets with multiple transactions. This study proposes an unified model with multi transaction based Security Constrained Optimal Power flow for the assessment of ATC and Congestion Management in deregulated system. It uses an AC load flow model and Successive Linear Programming (SLP) approach enforcing operating constraints imposing limits on both loading of transmission lines (either in MW or MVA) and bus voltage limits. It also enforces security constraints arising out of line outages and generator outages. The use of control such as generator bus voltages, reactive powers of switchable VAR sources and tap setting of on-load tap changing transformers enhances ATC and gives an improved Congestion Management solution. Two possible market policies, policy 1 with fixed fractions assigned to participants and policy 2 with flexible fractions are considered. A computer package has been developed in MATLAB and its effectiveness of the proposed method has been verified by solving modified IEEE 24 bus Reliability Test System and a 119 bus Indian Utility System. The results demonstrate the monotonic and reliable convergence of the proposed method satisfying effectively all the operating and security constraints. Security constrained ATC values and assigned transactions in Congestion Management obtained are enhanced due to the use of system controls.