A carpenter knife with locking member having a handle having a left shell and a right shell coupled together, a slit defined in the left shell, a stage formed rounding the slit; a knob being roughly of oval shape and having a central opening; a blade having a cutting edge and contained in and slidable with respect to the handle, an installing notch defined in the blade on a side opposite to the cutting edge; a blade seat contained in the handle, the blade seat having a bottom plate, two side walls, a spring tongue, and at least one installing protrusion, a top protrusion and a side protrusion formed on a free end of the spring tongue; and, a pin passing through the central opening of the knob and having a hole in which the top protrusion being firmly inserted in.