The present paper attempts to study the selection and related resonance phenomenon of dendritic growth with oscillatory external sources. As a prototype problem, we consider dendritic growth from a pure undercooled melt with the interference of a laser beam. The laser beam is assumed to act at the tip of the dendrite during dendrite growth, which introduces an external point heat source at the tip, and, as a consequence, creates a tiny hot spot there. The strength of laser beam is easy to control. Hence, the effective area of the nose spot created at the tip is also controllable. Moreover, this nose spot can be made to be steady, or oscillatory with a given frequency. For the case of steady laser beam, we derive the quantization condition and selection mechanism of dendrite growth depending on the effective area of the nose spot. We find that the steady laser beam may considerably affect the growth speed of dendrite. For the case of an unsteady laser beam, we deduce that the modulation frequency of the laser beam may induce a resonance with the dendritic growth.