Transmission lines play an important role in providing all participants the access to supply and demand in a competitive electricity market. Market size could be limited by the physical ability of transmission network to deliver electricity. Transmission line congestion as a primary indicator for transmission expansion planning has been investigated in this paper. The calculation on congestion cost under single side bid Poolco market is proposed and is defined as the social surplus difference between before the transmission line violation and after congestion. The generator's bidding price change is considered as a result of market factors in a deregulated electricity market. Finally, a numerical example is applied on IEEE Reliability Test Systems (RTS) for illustrating the essential features of the proposed congestion cost evaluation approach. The results are conducted using the well-known Monte-Carlo Simulation method.