Abstract: The polygalacturonase activity and some physiological changes of transgenic (with antisense PG gene) and control tomato fruits were systematically compared at green-mature, turning, pink-top, pink and red stages in ripening process. The results showed that PG activity of transgenic tomato fruits was lower than that in control fruits in ripening process. The PG activity of transgenic fruits were almost inhibited at red stage. Transgenic fruits are better than control fruits in firmness and storability. Synthesis and accumulation of lycopene of transgenic fruits are postponed, soluble pectin content and electric conductivity were all lower than those in control fruits. Stimuli of exo-ethylene caused sharp increase of PG activity in control fruits, but not in transgenic fruits. The interrelation of PG activity with softness, storability and susceptivity to stimulus from exo-ethylene was discussed. We proposed for the first time, the PG activity sharp increasing in control fruits and almost inhibiting in transgenic fruits were only at red stage, not at whole ripening stage, and the PG activity plays an important and direct role in fruit ripening softness.