Chemical strengthening is considered as the most suitable method for strengthening thin glass sheets used in mobile phones. The spray method of chemical strengthening requires a slurry to be sprayed on the glass sample to be strengthened. This slurry is prepared by mixing various compounds. In this study, the influence of α-Al2O3 particle size in the slurry on the physical properties of the chemically strengthened glass prepared by the spray method was investigated. The compressive stress (CS) was dramatically enhanced as the particle size of Al2O3 in the slurry decreased. The glass sample with 13 nm α-Al2O3 including the KNO3-Al2O3 slurry exhibited the highest CS of 905 MPa, and the depth-of-layer (DOL) of this sample was 37 μm. The same slurry composition also achieved the highest bending strength of 640 MPa under different heat treatment conditions. The optimization of the heat treatment conditions, such as temperature and time duration, resulted in the highest value of CS (916 MPa) obtained for the sample heated at 400 °C for 1.5 h, and the maximum DOL (65 μm) was obtained for the sample at 480 °C for 4 h.