Abstract: The effects of heat stress on physiology of two lily cultivars at seedl ing stage were studied. The results show that the contents of pigment and soluble protein decrease continuously. The CAT activity and proline content show a transient rise and then reduce. Genotype ‘K_1-4’having better heat tolerance shows a less fall in pigment and soluble protein contents, higher CAT activity and higher proline content. Those physiology indexes of the cultivar ‘Avita’ being sensitive to heat are worse.