Overlapping extension PCR was used to replace the amino acid residue His 297 (H) by Phe (F) near the activity center of XynA from Geobacillus stearothermophilus. XynA and XynAH297F were separately expressed in Escherichia coli BL21 (DE3) and were both purified and characterized. The optimum pH for XynAH297F was 5.5, which was similar to that for XynA. After treatment in pH4-10 buffers over 20hs, the residual activities of XynA and XynAH297F were both above 90%, which indicated that they had good pH stability. The optimum temperature for XynAH297F (75 ℃) was increased compared with XynA (70 ℃). In addition, the XynA and XynAH297F respectively remained about 10% and 30% residual activities after incubated at 80 ℃ for 20 min. It proved that the thermal stability of XynAH297F was improved at certain degree. Compared with XynA, XynAH297F displayed increases in Km, Vmax and Kcat. The results showed that the affinity of enzyme to substrate was decreased, but the catalytic rate was increased.