Laser beam transformation utilizing the effect of multimode interference in multimode (MM) optical fiber is thoroughly investigated. When a Gaussian beam is launched to an MM fiber, multiple eigenmodes of the MM fiber are excited. Due to interference of the excited modes, optical fields that vary with the MM fiber length and the signal wavelength are generated at the output facet of the MM fiber. Diffractive propagation of these confined fields can yield various desired intensity profiles in free space. Our calculations show that, an input fundamental Gaussian beam can be transformed to frequently desired beams including top-hat, donut-shaped, taper-shaped, and low-divergence Bessel-like within either the Fresnel or the Fraunhofer diffraction range, or even in both ranges. Experiments on a monothic fiber beam transformers consisting of a short piece of MM fiber (∼ 10 mm long) and a single-mode signal delivery fiber were carried out. The experimental results indicate the functionality and high versatility of this simple fiber device. The performance of this fiber device can be easily and widely manipulated through parameters including the ratio between the core diameters of the SM and MM fiber segments and the length of the MM fiber segment. In addition, the intensity profile of the output beam can be controlled by tuning the signal wavelength even after the fiber device is fabricated. Most importantly, this technique is highly compatible with the technology of high power fiber lasers and amplifiers and fiber delivery systems.