A master cylinder of a brake is disclosed. The master cylinder includes a cylinder body connected with an oil tank and defining therein a first pressure chamber and a second pressure chamber, which are separated from each other by a partition, a first piston to perform reciprocating movements in the first pressure chamber, a second piston to perform reciprocating movements in the second pressure chamber, a first return spring to elastically support the first piston in the first pressure chamber, and a second return spring to elastically support the second piston in the second pressure chamber. The first piston has a plurality of recesses formed at one end thereof to assure smooth supply of oil during braking. The master cylinder has the effects of assuring smooth supply of brake oil during braking, and preventing fluctuation of a piston therein.