Power producers' profits are determined by the market price in the electricity market and therefore they will adopt certain strategies in market transactions to achieve higher profits. In an electricity spot market that adopts uniform pricing

power producers with considerable generation capacity are able to exercise their market power

given that the market concentration is relatively high at the beginning of market reform. It has been proved that an effective bidding strategy can increase the market clearing prices

so as to increase the profits of the power producer. Fortunately

the introduction of long-term transactions may mitigate the impact of producers' market power

as a great amount of long-term volume is settled at the long-term contract price

which is determined in advance and is less fluctuating than the spot price. Two forms of long-term transactions

the fixed volume contract and the varied volume contract

are studied in this paper. Simulation studies are conducted on a multi-agent platform

where both the long-term and spot transactions of power producers are included

and the total profits of a power producer with or without long-term transactions are analyzed to demonstrate their influence. Meanwhile

the results clearly show that long-term transactions can effectively prevent power producers from exercising market power.