A novel and simple controlled all-optical ON/OFF switch using the gain modulation in a single-mode Fabry-Perot laser diode (SMFP-LD) is demonstrated. The working principle of the proposed ON/OFF switch is explained and the experimental results with 10 Gb/s data signal are presented. The 10 Gb/s data signal is switched ON and OFF with a 350 MHz control signal. The extinction ratio of 15.6 dB is observed at the output port with a 2 dB power difference between the control signal and input signal. We measured the rising and falling time of the switch output as 48 and 59 ps, respectively.