Whether oil futures market can perform price discovery function well is very important in global economics and energy markets. The interaction between oil spot and futures prices exists due to intraday information transfer and arbitrage trading. However, the traditional methods used in price discovery analysis ignore the interaction, and thus introduce the biased conclusions. This paper uses simultaneous equation analyze the interaction effect between oil spot and futures returns, estimates the model by the method of modified identification through heteroskedasticity (modified ITH) and examines price discovery function of oil futures markets. Using weekly spot and futures prices of Brent crude oil, gas oil and heating oil between Feb 12, 1999 and Jan 30, 2009, the results suggest oil futures return will affect the corresponding oil spot return. The unidirectional interaction exists. This indicates the information will transfer from futures markets to spot markets and oil futures markets have the major price discovery function. This paper also offers a new view of examining price discovery, i.e. interaction effect.