Based on the fiber mode theory,the electromagnetic wave propagation characteristics of the three-layer flattened mode micro/nano fiber have been investigated. The influences of the index and core size on the waveguide dispersion have been studied detailedly. Numerical simulations show that the dispersion of a flattened mode micro/nano fiber has different properties from conventional micro/nano fiber and flattened mode fiber. In the wavelength range of 0.3-1.6 μm,with the variation of the index,the waveguide dispersion curve′s only minimum point increases or decreases regularly,and show red shift or blue shift. While with the variation of the core size,the waveguide dispersion′s minimum point reduces obviously,but the minimum point is located at the same position of 0.6 μm. Whether the changed condition is the index or core size,in 1.3-1.6 μm band,the waveguide dispersion increases slowly and its value tends to zero gradually. When the core size changes,this trend gets more significant.