Fresh rhizomes of Ligusticum chuanxiong, a commonly used traditional Chinese medicinal herb, were collected monthly from a cultivating base in China practicing good agriculture practice (GAP). These samples were analyzed by HPLC-UV for their main chemical ingredients. Senkyunolide A (6), coniferyl ferulate (7) and Z-ligustilide (8) were identified as the major ingredients. The accumulation of the main ingredients with time in the herb was elucidated. Both individual and total contents of all main components gradually increased from the beginning of October to the middle of next April. The weight of a single rhizome reached a plateau at the end of May, whilst the content of the major ingredients peaked in the middle of April. Based on these results, it is recommended that Rhizoma Chuanxiong be harvested between the middle of April and the end of May.