It is known that no uniform electric field profile exists in a planar linear waveguide. The uniform core field can be shown to exist in symmetrical planar waveguides and circular fibers with nonlinear claddings. Theoretical analysis and numerical calculations are carried out to show that when the modal index equals to the core refractive index, the core field becomes uniform at an appropriate optical power. Analysis for a step index circular fiber with nonlinear cladding have also shown that the core field becomes uniform under similar conditions. The occurrence of a uniform field in a waveguide core may believe to have promising applications in waveguide, optoelectronic and photonic devices. The uniform core field can be shown in symmetrical planar waveguides and circular fibers with nonlinear claddings. Theoretical analysis and numerical calculations are carried out to show that when the modal index equals to the core refractive index, the core field became uniform at an appropriate optical power. Analysis for a step index circular fiber with nonlinear cladding have also shown that the core field became uniform under similar conditions. waveguide core may believe to have promising applications in waveguide, optoelectronic and photonic devices.