Abstract: The one-dimensional thermal conduction equation and Navier-Stokes equation are used to get the pulse stress wave in a semi-infinit medium when a rectangular pulse laser of t₀ duration is incident on its surface. For both transparent and opaque samples the ultrasound wave forms are discussed when the surface is free or clamped. Some experimental results on opaque sample are presented. It is shown that,theoretically and expetimentally, the ultrasound pulse thermoelastically generated by a pulse laser is bipolar when the sample surface is free and monopolar when the sample surface is clamped.