A master cylinder of a brake for a vehicle may include: a housing having a port through which oil is transferred; a motor connected to the housing so as to supply rotary power; a screw rotatably installed in the motor, and rotated by the rotary power received from the motor; a moving piston engaged with the outside of the screw, and moved in the longitudinal direction of the housing by the rotation of the screw; and a fixed piston fixed to the inside of the housing, and having the moving piston positioned on the outside thereof.