We study the problem of pricing arithmetic Asian options when the underlying is driven by stochastic volatility models with two well-separated characteristic time scales. The inherently path-dependent feature of Asian op-tions can be e ciently treated by applying a change of numeraire, introduced by Vercer. In our previous work on pricing Asian options, the volatility is modeled by a fast mean-reverting process. A singular perturbation expan-sion is used to derive an approximation for option prices. In this paper, we consider an additional slowly varying volatility factor so that the pricing par-tial di erential equation becomes four-dimensional. Using the singular-regular perturbation technique introduced by Fouque-Papanicolaou-Sircar-Solna, we show that the four-dimensional pricing partial di erential equation can be ap-proximated by solving a pair of one-dimensional partial di erential equations, which takes into account the full term structure of implied volatility.