The introduction of an electricity pool in restructuring the power industry realizes efficient pricing of electricity on the principle of a market mechanism, but it also brings the risk of price volatility to power companies and customers. To analyze price volatility in the pool, we use an approach that expresses the variations in supply/demand curves by a few parameters and evaluates the quantitative correlation between these parameters and factors such as expected demand and fuel cost. The case study of the day ahead market in California clarified that variation in the supply curve can be decomposed into one that correlated with expected demand and another that correlated with fuel cost. The quantitative analysis of variation in supply/demand curves can be applied to the studies on the evaluation of price risk and market power in an electricity pool.