This paper proposes a new approximation formula for average options on commodities under stochastic volatility environment. In particular, it derives a formula under two stochastic volatility models such as Heston and λ-SABR models including the SABR model as a special case by using an asymptotic expansion method.To our knowledge, this paper is the first one that shows an analytic (approximation) formula under stochastic volatility models for valuation of average options structured for commodity contracts. Then, it confirms its sufficient accuracy through numerical examples. It also implements calibration to the WTI futures option market that is one of the most liquid commodity markets. Using the parameters obtained by calibration, it compares model-based prices with those of traded average options in NYMEX.