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This paper describes a laser optical technique that allows the detection of in-plane motion of Lamb waves. This interference-based laser optical technique includes a tiny square indentation with a width of about 30 /spl mu/ on the sample surface and a relatively simple optical arrangement. The current technique is applied for the detection of in-plane motions of Lamb waves propagating in a 70-/spl mu/. thick brass plate. Measurement of S/sub 0/ mode dominated by in-plane motion in the low fd (frequency times thickness) regime is successfully demonstrated with the current technique. With the indentation replaced by a microreflector in a microelectromechanical (MEMS) structure, this technique is applicable for the detection of in-plane motion in MEMS structures.