Dental zirconia implants fabricated by the mechanical machining and sintering of zirconia blocks have many surface cracks that lead to the deterioration of mechanical strength and the failure of the implant in the body. In this study, we attempted to manufacture an extremely dense and crack-free zirconia specimen by slip casting and pressureless sintering. After the preparation of zirconia slurry by control of its viscosity and by solid loading, highly dense zirconia specimens could be obtained by pressureless sintering at 1450 °C for 2 h. Slurry viscosity was controlled by adjusting the mixing ratio of 3Y-TZP powder, a dispersant, and a pH adjustment agent. Highly dense 3Y-TZP specimens with a relative density of 99% and small grain size of 200-400 nm could be obtained at a solid loading of 50-65 wt%. An optimally dense specimen was fabricated from zirconia slurry with 60 wt% solid loading that had the highest apparent density of 6.07 g/cm³ (99.5%).