An all-optical microwave oscillator is proposed and experimentally demonstrated, in which a distributed feedback laser diode (DFB-LD) acts as light source and optical-optical modulator simultaneously. By employing stimulated Brillouin scattering (SBS) process for oscillation frequency selection and active optical comb filter for side-modes suppression, single-mode photonic microwave signal generation can be achieved through a pure optical oscillation. In the experiment, a stable 10.87-GHz photonic microwave signal is obtained. The signal quality is evaluated by measuring the side-mode suppression ratio and single side-band phase noise of the signal. The corresponding values are 42 dB and −85  dBc  /  Hz at 10 kHz, respectively. This scheme explores the innovative application potentiality of DFB-LD, which will benefit on-chip photonic microwave systems.