The positron lifetime (PLT) in a thin film of amorphous hydrogenated carbon (a-C:H) was studied by a pulsed positron beam. For the first time the PLT was measured in a layer which is as thin as 100 nm. PLTs of 400(5) ps for the surface state and 348(3) ps in the volume of the film were obtained. The diffusion length of the positrons was measured to be only 4(1) nm. From these results we conclude that in the specimen investigated at least 3