After the introduction of an electricity pool, it would be important for power companies and customers to evaluate the risk of price volatility by assuming that it follows a stochastic process. With the use of a supply/demand curves model, we showed the process of evaluating electricity price risk in a day-ahead market when supply-demand structure in the market has changed. By applying this process to the data in CalPX, we explored the difference in price risk between perfect competitive condition and imperfect competitive condition, and analyzed the price risk when price responsiveness of demand changed.