By constructing the expression system for fusion protein of GFPmut1 (a green fluorescent protein mutant) with the hyperthermophilic xylanase obtained from Dictyoglomus thermophilum Rt46B.1, the effects of temperature on the fluorescence of GFP and its relationship with the activities of GFP-fused xylanase have been studied. The fluorescence intensities of both GFP and GFP-xylanase have proved to be thermally sensitive, with the thermal sensitivity of the fluorescence intensity of GFP-xylanase being 15% higher than that of GFP. The lost fluorescence intensity of GFP inactivated at high temperature of below 60A degrees C in either single or fusion form can be completely recovered by treatment at 0A degrees C. By the fluorescence recovery of GFP domain at low temperature, the ratios of fluorescence intensity to xylanase activity (R (gfp)/A (xyl)) at 15A degrees C and 37A degrees C have been compared. Even though the numbers of molecules of GFP and xylanase are equivalent, the R (gfp)/A (xyl) ratio at 15A degrees C is ten times of that at 37A degrees C. This is mainly due to the fact that lower temperature is more conducive to the correct folding of GFP than the hyperthermophilic xylanase during the expression. This study has indicated that the ratio of GFP fluorescence to the thermophilic enzyme activity for the fusion proteins expressed at different temperatures could be helpful in understanding the folding properties of the two fusion partners and in design of the fusion proteins.