Controllable size Pd -Ni alloy nanoparticles with Ni8.5 % were electrodeposited on carbon fiber by using -0.5 V potential. The nanoparticles -coated carbon fibers could be applied for sensing hydrogen. The size of the nanoparticles affected the hydrogen sensing property of the sensor. The nanoparticles with 180 nm diameter showed good hydrogen sensing property. The response current of the sensor increased with hydrogen concentration from 0 to 6%. In 6% H 2, the sensitivity of the nanoparticle sensor reached 38.35%.