Abstract: The effects of 5-azacytidine（5-azaC）on DNA methylation levels and heat tolerance of two commercial cultivars of non-heading Chinese cabbage [Brassica campestris L. ssp. chinensis（L.）Makino]，e.i. ‘Xiadi’and‘Dongshangwei’，were studied. The results showed that DNA methylation levels of seedlings of the two cultivars were significantly decreased，no abnormal growth or development was observed. Pre-treatment of seeds of non-heading Chinese cabbage with 0.1 ~ 0.2 mol · L^-1 of 5-azaC would slow down the decreasing of growth，POD activity，and soluble protein content of seedlings under heat stress，and decrease MDA content and cell membrane permeability as well.

Key words: non-heading Chinese cabbage, 5-azaC, DNA methylation, heat stress, heat tolerance