Argon‐laser‐pumped single‐transverse‐mode laser operation was obtained with a three‐dimensionally confined LiNdP4O12slab waveguide laser, in which one endface was cylindrically polished and directly coated with a laser mirror. The threshold, the output power, and the oscillating transverse modes were theoretically analyzed, both for transverse pumping and for longitudinal pumping, considering the spatial distribution of the pump beam and the laser beam. The necessary conditions for the pump sources to obtain a TE00single‐transverse‐mode oscillation were determined on the supposition of the diode pumping.