Stainless steel is an alloy of steel which contains at least 10.5% chromium, less than 1.2% carbon, and other alloying elements and it is widely used in many industries globally and their properties are highly influenced by their microstructure, heat treatment or by plastic deformation. But due to hardness, poor wear, and corrosion resistance, leading to short service life, there is need to investigate the effect of annealing on the mechanical properties of alloy 304H stainless steel and how the mechanical properties can be improved with a view of improving its service life and optimizing engineering usage. Sixteen (16) samples of the alloy were used. Twelve (12) samples were annealed at three different temperatures of 950oC, 1000oC and 1050oC inside a muffle furnace. At each temperature four samples were heat-treated inside the muffle furnace for 30 minutes. The result showed that the yield strength decreased from un-annealed sample to annealed samples at 950oC with a value of 504.8MPa and increased at 1000oC with a value of 610MPa and a decrease of 323.8 MPa was obtained at 1050oC. Also, the ultimate tensile strength showed an increase from   950oC with a value of 826.3MPa to 1000oC with a value of 930MPa but there was a slight decrease at 1050oC for all samples. The ultimate tensile strength at 1000oC with a value of 930MPa was the highest in all the samples. The annealed samples at 1000oC had the highest percentage elongation of 13.57% which shows an increase in the ductility of the material. The hardness of the material decreased from 157.25 BHN at 950oC to 134.00BHN at 1000oC. An increase to 169.50BHN was however obtained at 1050oC. Thus, full-annealing of alloy 304H stainless steel at 1000oC increases in ductility as hardness decreases.