The diffraction of a solitary wave by a thin wedge with vertical walls is studied when theincident solitary wave is directed along the wedge axis. The method of multiple scales is extended to thisproblem and reduces the task to that of solving the two-dimensional KdV equation with proper boundary andinitial conditions. The finite-difference numerical procedure is carried out with the fractional step algorithmin which difference schemes are all implicit. Except the maximum run-up at the wall, the results in this paperare found to corroborate the Melville’s experiments not only qualitatively but also quantitatively. Themaximum run-up of our results agrees well with Funakoshi’s numerical one but it is considerably larger thanthat in Melville’s experiment. An important reason for this discrepancy is believed to be the effect of viscousboundary layer on the vertical side wall. The diffraction of a solitary wave by a thin wedge with vertical walls is studied when theincident solitary wave is directed along the wedge axis. The method of multiple scales is extended to thisproblem and reduces the task to that of solving the two-dimensional KdV equation with proper boundary and initial conditions. The finite-difference numerical procedure is carried out with the fractional step algorithmin which difference schemes are all implicit. Except the maximum run-up at the wall, the results in this paperare found to corroborate the Melville’s experiments not only qualitatively but also quantitatively. Themaximum run-up of our results agrees well with Funakoshi’s numerical one but it is mainly larger thanthat in Melville’s experiment. An important reason for this discrepancy is believed to be the effect of viscousboundary layer on the vertical side wall.