The effects of melt pool geometry on crystal growth and microstructure development during laser surface melting of single-crystal alloys were studied by means of mathematical modeling and experiments. A mathematical model was developed for the three dimensional (3-D) melt-pool geometry and single-crystalline melt-pool solidification in laser surface melting. The 3-D melt pool geometry is described by four geometrical parameters (w, l, h, a). The model was used to study the effects of variations in the geometrical parameters on crystal growth and microstructure development in the melt pool. Laser surface melting experiments with single crystal nickel-base superalloys were conducted to verify the computational results of microstructure development in the melt pool. Results indicate that the melt-pool geometrical parameters have profound influences on the dendrite growth velocity and growth pattern in the melt pool. For the (001)/[100] substrate orientation, variations in l/w and a can influence both the number and the relative sizes of growth regions while the variation in h/w can only influence the relative sizes of the growth regions. Unidirectional dendrite growth along the [001] crystallographic direction can be achieved for an a value of 45° or below. The maximum dendrite-growth velocity in the melt pool is related to a and l/w, but independent on h/w. Experimental microstructure observations agreed well with the computational results.