We prepared Ag-sheathed Bi2Sr2CaCu2O8 (Bi2212) tapes heat-treated in high fields (in-field heat-treatment Bi2212) and heat-treated without magnetic fields (out-of-field heat-treatment Bi2212), in order to examine the magnetic field effect on the microstructure of Bi2212. The differential thermal analysis (DTA) was performed at 10 T using Bi2212 powders. The DTA suggests that the in-field heat-treatment changes the grain morphology of Bi2212. It was found that the critical current density Jc for the in-field heat-treatment Bi2212 tape is largely improved at 10 K in fields. In addition, the in-field heat-treatment Bi2212 tape has also a large n-value in the form of E=Ec(J/Jc)n, which is related to the microstructure change.