All-optical decision gate based on two-mode injection locked Fabry-Perot lase r diode (FP-LD) and its application in decision on a distorted 10Gbit/s return-to-zero (RZ) p seudorandom bit sequence(PRBS) are studied in this paper.Firstly,the transfer characteristics of the injection and the output power in two-mode injection locked FP-LD are experimentally studied.The “S-shape” transfer characteristic curve of test result shows that the structure of two-mode injection locked FP-LD has the ability for all-op tical decision on injection optical data signal.Furthermore,all-optical decision on a 10Gbit/s RZ optical PRBS is reali zed by properly adjusting the polarization state of the injection data signal and the detuning between the inj ec tion data light and the closest free running longitudinal mode of FP laser.The result of decision is observed throug h optical oscilloscope.Compared with the eye diagram of the distorted injection RZ data,the noise added to zero code is suppressed,and the eye diagram after decision is obviously opened.So the feasibility of all-optical d ecision on high speed optical signal by using two-mode injection locked F P-LD is verified,which shows a vast application prospect to the field of all-o ptical signal processing.