Aloysia triphylla (L'Her.) Britton (Verbenaceae) is an aromatic shrub native to Argentina and Chille; known as herb louisa or lemon verbena. It is an important medicinal and aromatic plant that contains high quality essential oil of lemon scent that has an antifungal activity, and is used as a sedative, antispasmodic, anti diarrhea, and to add flavor to food and beverages. A study was carried out during two successive seasons of 2005/2006 and 2006/2007 at the Experimental Station of Medicinal and Aromatic Plants, Fac. Agric., Mansoura Univ. The research aimed to study the effect of drying method (shade and oven) and the length of storage period (2, 4 and 6 months) on essential oil percentage and composition of the leaves of two harvesting dates (June and September). The physical and chemical properties of the oil were determined and are included in this study.  The results showed that the oil percentage obtained from September harvest was higher than that of June harvest in both seasons.  Shade drying gave the highest oil % followed by oven drying method at 45ْC compared to the fresh leaves.  In both June and September cuts, increasing storage period of the dried leaves of lemon verbena decreased the essential oil percentage in both drying methods.   The G.L.C. of the essential oil of the fresh leaves revealed a total of 20 compounds. The total identified compounds constituted 83.5 % and 76.8 % in the oil of shade and oven dried leaves, respectively in June cut, while constituted 87.9 % and 85.9 % in the same order in September oil.  The most important compounds in both cuts were 1,8-cineole, geranial, limonene, linalool, and spathulenol and accounted for 49 % of  June oil, while summed 54.3 %  in September.  The method of drying affected the proportion of the oil components in the two cuts.  The relative concentrations of each constituent changed with the length of the storage period. The percentage of some compounds increased (linalool and spathulenol), others decreased (1, 8- cineol, geranial, and limonene), while others changed slightly in their percentages (geranyl acetate and menthone).  Some compounds were not present in the oil of June (α-pinene, camphene, citronellal, ocimene, and sabinene), while were present after storage.  In September oil, citronellal and curcumene were found in the control, but disappeared after storage.