Abstract: The paper reported that ABA can accelerate the senescence of excised needles of Pinus Koraiensis, on basis of the evidences that RNase activities were increased distinctly, the yellowing of needles intensified and the ultrastructures of mesophyll cells senesced sequentially by treatment of excised needles with ABA. According to the order of the degree of sensibility to senescence of different organelles in mesophyll cells of needles of Pinus Koreiensis, we divide the process of senescence into three periods:the early stage of senescence, the middae stage of senescence and the late stage of senescence and sum up as follows:Start of senescence#br# Chloroplasts (envelope membrans degraded, lamellas partly swolled) Endoplasmics (vesicle appeared) False plasmolysis (appered)#br# Chloroplasts (envelope membranes damaged, a large of lamellas spoiled) Mitochoadrias (envelop membranes swolled, No, of cristaer.decreased) Endoplasmics (hyperplasia temporally) Golgi osomes (s wo lied) Primary lysosomes (formed) Nuclear pores (closed)#br# Chloroplasts (disintegrated completely) Mitochondrias Endoplasmics Golgiosomes (disintegrated) Membranes of {vacuoles lysosomes drotoplasms (disintegrated)#br# Cells died.