Provided is a forming method of a metal layer suitable for a 3D printing process. The method includes the steps of providing a plurality of metal particles on a substrate; applying an oxide-removing agent to the metal particles to remove metal oxides on the metal particles; at a first temperature, performing a first heat treatment on the metal particles for which the metal oxides are removed to form a near shape; and at a second temperature, performing a second heat treatment on the near shape to form a sintered body. The first temperature is lower than the second temperature.