Abstract:The leaf structure, amaranthin and heat stable protein contents, as well as photosynthetic characteristics were studied in leaves of two cultivars of Amaranthus tricolor L., such as ‘Red flower’and ‘Green leaf’, under the artificial thermal stress of 40℃/35℃ (day/night) for 6 days. The results showed that the amaranthin accumulated greatly in leaves of ‘Red flower’, its content was 1.81 times of that in ‘Green leaf’under high temperature for 6 days. Under light microscope, amaranthin distributed in lower and upper epidermis, vascular bundle of ‘Red flower’, but did not appear in ‘Green leaf’. The palisade tissue structure remained normal in ‘Red flower’, but it partially discontinued and chloroplasts damaged seriously in ‘Green leaf’after 6 days under high temperature stress. Compared with ‘Green leaf’, the chlorophyll and carotenoids contents in ‘Red flower’decreased slightly, the content of heat stable proteins slightly rise, and its light compensation points and saturated light intensity obviously delay. After high temperature stress, the photosynthetic parameters, such as net photosynthesis rate (Pn), intercellular CO2 concentration (Ci) and stomatal conductance (Gs) in 'Green leaf’were significantly lower than those in ‘Red flower’(P<0.01). It suggested that the accumulation of amaranthin was closely correlated with the thermostability of ‘Red flower’under high temperature, which was benefited to maintained a stable photosynthetic performance.