We have synthesized nanometer-sized PbS clusters embedded in a new kind of polymer(Flemion) film. Their linear optical absorption spectrum was blue-shift from the bulk band-gap but no exciton peaks are abaerved. Using femtosecond laser pulses, we have recorded the time-resolved Kerr transmission signals of the films, from which we got the third-order nonlinear susceptibility and response time. The experimental data are discussed in terms of the quantum confined effect.