Uniform electric field exists only in nonlinear waveguide structures. Theoretical analysis and numerical results for typical examples are presented for three-layer nonlinear slab waveguides to find the appropriate structures in which the core electric field can become uniform at appropriate optical power. It is shown that there are five kinds of such structures. The electric field profiles are plotted for five typical examples. We notify that the occurrence of uniform field in a waveguide core may perhaps have prospective applications in waveguide, opto-electronic and photonic devices.