The tea remains green because the oxidation process is stopped before it can begin by heat. Chinese and Japanese Green Teas are processed in a different way, giving each their distinctive characteristics:

Chinese Green Tea: The fermentation process is first stopped by sunlight or warm air to the leaves. Next the leaves are pan-fired which stabilizes the flavor and the fragrance of the leaves (but will change the color of the leaves). The way the leaves are handled or moved is distinctive to each region and processor, and affects the outcome of the tea. Generally the processing is still done by hand in China.

Japanese Green Tea: The Japanese usually apply steam to stop the fermentation process. For higher quality "every day" Green Tea or Sencha the leaves are rolled and twisted and then dried. Hojicha is then roasted giving it a nutty flavor For the highest quality Japanese green tea - Gyokuro - the leaves are dried after steaming and then rolled and twisted. The bushes used to grow the tea for Gyokuro are covered for three weeks to help the plant make more chlorophyll resulting in a very green color. These leaves are plucked only once a year.