Abstract Although striations and dross are the most significant quality factors in laser cutting operations, the mechanics of their formation is not yet entirely clear. In this paper striation and dross formation are analysed by means of an analytical model which, by considering mass, force and energy balances, evaluates the 3D geometry of the cutting front, and the geometry and temperature fields of the melt film. On this basis, an interpretation for the striation pattern based on the evolution of the melt film is proposed, thus allowing prediction of the well-known transition from single slope at low process speeds to double slope at high speeds. In the same way, the effect of the assistant gas pressure is predicted, in accordance with experimental observations by many authors. The mechanics of dross formation is also discussed, by introducing the kinetic energy of the melt film and relating it to the local temperature.