The present disclosure relates to garnet powder, a manufacturing method thereof, a solid electrolyte sheet using a hot press, and a manufacturing method thereof. In particular, the present disclosure provides a method for manufacturing Li7La3Zr2O12 (LLZ) garnet powder including preparing a mixture by first dry mixing Li2CO3, La2O3, ZrO2, and Al2O3. The mixture is first calcinated for 5 to 7 hours in a temperature range of 800 to 1000° C. The calcinated mixture is ground to a powder with an average particle size of 1 to 4 μm through dry grinding. A cubic-phased LLZ garnet powder is prepared by second calcinating the ground mixture for 10 to 30 hours in a temperature range of 1100 to 1300° C.