We have designed and fabricated a quantum dot (QD) gain medium which consists of InAs QDs in an InGaAsP matrix on an InP substrate. By using these InAs/InP QD layers, we have generated femtosecond (fs) pulses with pulse duration of 295 fs from a single-section monolithic Fabry-Perot (F-P) cavity at the repetition rate of 50 GHz around 1560 nm wavelength range without any external pulse compression. The average output power is 40.1 mW at the injection current of 200 mA. Optical signal-to-noise ratio (OSNR) of the proposed QD mode-locked laser (QD-MLL) is up to 50 dB. The lasing threshold current and the external differential quantum efficiency are 23 mA and 30 %, respectively. And the mode beating linewidth was measured to be less than 20 KHz. We have interpreted that several nonlinear optical effects related to interaction of QD excitons with intracavity laser fields could create nonlinear dispersion to compensate intracavity linear dispersion. So total dispersion is minimized and four-wave mixing (FWM) is dramatically enhanced within QD F-P cavity. If spectral bandwidth is broad enough, tens or hundreds of longitudinal modes would lase and their phases would be locked together through FWM process. Eventually a train of fs pulses with a repetition rate corresponding to cavity round-trip time is generated.