Due to our previous research, mainly the thermostable mutants Q307D, Y311K, and I427L, we conjectured that Escherichia coli AppA phytase's C-terminal plays an important role in its thermostability, and AppA begins to collapse from the C-terminal when at a higher temperature. So here we constructed C-lose mutant to prove it. The residual activities of the wild-type AppA phytase and C-lose were 31.42 and 70.49 %, respectively, after being heated at 80 A degrees C for 10 min. The C-terminal deletion mutant C-lose showed 39.07 % thermostability enhancement than the wild-type both without the pH and temperature optimum changed. It proved the C-lose plays a key role in E. coli AppA phytase's thermostability.