The small signal gain measurement of the all gas-phase iodine laser driven by D2-NF3-DCI combustion is taken using a wavelength tunable 1315 nm diode laser gain measurement system. A small signal gain of 3×10-5 cm-1 and the profile of small signal gain along the flow direction are obtained. The positive gain indicates that a material progress has been made in the D2-NF3-DCl combustion-driven all gas-phase iodine laser,but it is still difficult to demonstrate lasing as to the magnitude of the small signal gain,a detailed parameter optimization must be done in order to improve markedly the small signal gain,and the plentiful supply of HN3 and the optimization of the supersonic mixing flow field are key factors to obtain a high small signal gain for the D2-NF3-DCl combustion-driven all gas-phase iodine laser.