Different from western markets, the margin rates in Chinese futures markets are raised when contract approaches maturity. This paper concentrates on the effect of this time dependent margin rule on volatility. Open interest, another candidate in the margin rule, is also included in our model to investigate its necessity as one of the factors of the rise of margin rates. With the popular copper contract in Shanghai Futures Exchange (SHFE) , our test results suggest that margin levels have a significant positive effect on volatility, yet open interest has little to do with volatility. The implication is that the rise of margin rate approaching maturity virtually deteriorates the degree of market risks, and open interest is not a necessary factor for the margin rule. It indicates that the policy tool, represented by margin rates, has significantly greater influence on volatility than the market element, represented by open interest.