Mushrooms are very popular in the market for their nutritional and medicinal use. Mushroom volatiles are not only an important factor in the flavor, but also contain many antioxidant compounds. Antioxidant activity is a very important property for disease prevention. The volatile compositional characteristics of straw mushrooms (Volvariella volvacea [Bull. ex Fr.] Sing.) and oyster mushrooms (Pleurotus ostreatus [Jacq. ex Fr.] Kummer) during maturity and the mushroom antioxidant activity related to the non-volatiles and volatiles are studied by a chromatographic method in combination with a spectrophotometric method. The volatile compounds of straw and oyster mushrooms are sampled and identified by a combination sampling method, including headspace solid phase microextraction and steam distillation, followed by gas chromatography-mass spectrometry detection. Among all the volatile compounds identified, 1-octen-3-ol and 3-octanone are the two main compounds with the highest amounts in the volatile compositions of straw and oyster mushrooms. During maturity time of the straw mushrooms, the unsaturated 1-octen-3-ol peak area is reduced, whereas the saturated 3-octanone peak area is increased. However, during normal maturity time of oyster mushrooms, the peak areas of 1-octen-3-ol and 3-octanone remain at the same level. 1-Octen-3-ol has a different antioxidant activity from 3-octanone. Combining the results of antioxidant experiments of water extract and main volatile components by the use of a phosphomolybdenum spectrophotometric method, the conclusion is drawn that oyster mushrooms might possess stronger antioxidant activities than straw mushrooms.