The experiment investigated the effect of storage period on spearmint herb oil composition. Spearmint herb samples were dried under shade for seven days and kept in polyethylene bags and stored at ambient room temperature for 12 months. Spearmint oil was extracted by hydro distillation using the technique of British Pharmacopoeia. Oil composition investigated every four months; initially in January before herb storage, April, August and December; to determine oil constituents using Gas Liquid Chromatography (GC) provide by Mass Spectrometry (MS). The results showed variation in oil constituents with progress of storage period. Carvone content slightly increased from 72.0% at January to 84.8% at April during the first four months and progressively decreased to 54.7% at the end of the storage period. The different oil constituents differed greatly during storage of spearmint herbs.