A lamb wave propagation behavior on a freestanding nanoscale membrane was investigated using a laser ultrasonic technique in the present study. A 110-nm thick aluminum (Al) layer was deposited on a rectangular 200-nm thick silicon nitride (SiN) membrane and the Lamb wave was launched using a pulsed laser. The transfer matrix technique was employed to obtain a theoretical dispersion curve so that material properties of the SiN membrane could be estimated through curve-fitting. In addition, picosecond ultrasonic measurement was used to characterize the Al film. Results showed that the dispersive behavior of Lamb wave in the fundamental antisymmetric mode could be clearly observed on the membrane. However, comparison of dispersion curves indicated that the effect of residual stress of the film became more influential at a low dimensional scale.