A single- and dual-wavelength fiber laser with multi-transverse modes is proposed. Mode interference is realized in the core of an optical fiber by writing a long period fiber grating on a few-mode fiber to obtain the LP01 mode and the LP11 mode simultaneously. A mode interferometer based on a few-mode long period fiber grating (FM-LPFG) is used as a comb filter in the ring-cavity fiber laser. Single- and dual-wavelength outputs can be achieved in the fiber laser by adjusting a polarization controller (PC). A mode-selective photonic lantern is used to realize mode conversion, and six LP modes, LP01, LP11a, LP11b, LP21a, LP21b, and LP02, can be generated. A single- and dual-wavelength fiber laser with multi-transverse modes can be achieved by combining a mode interferometer with a mode-selective photonic lantern. This work has potential applications in mode division multiplexing (MDM) systems to enlarge the capacity of optical communications.