The operating band with of a planar three-layered metal-dielectric waveguide (MDW) for a single mode operation on orthogonally -polarized waves is calculated dependity on the geometry and the material parameters of the MDW. and lower and upper middle layers (MLs) with dielectric permitivity 1 &3 (9 > iz2 2 s3) located between metal screens (MSs) may be applied in the integrated optic. devices [ 1,2] and in millimeter wave dielectric integrated circuits [3].The MDW bandwidth is determinated by the fiequency band (or by the band of wave numbers %) for a single-mode propagation on orthogonally polarized ET or E;" -waves ( ( ( x ) ) and (cy)) indexes indicate the direction of +he transverse component of the electrical wave vector, which is, respectively, para!lel or perpendicular to WL or MS surfaces). The band width is defined by the value of normalized wave number AAx, = A/$-& ,/= ( k, -the vacuum wave number, bl -the half of WL thickness), where Ak, , is determinated as a difference of k, values at the cutoff point of propagaLion of the fundamental ( E,"') and higher order (E;') modes. The wave numbers at the cutoff points of Et: -waves are found fiom the dispersion equations obtained for tree -layered MDW in [4]. Using normalized parameters, we can rewrite these equations as:' E:; -waves. Three -layered MDW, consisting of a waveguide layer(WL) with dielectric permitivity Mxlh( G@) + J W l h ( C & X ) .( 2Ax ) = (1),/= t h ( i $ F q ' &' = & , y J m / , / q 9 Eli = q/&i 9 '2,3 = b2,3/b1 7 B = (q -c3)/( izI -g2) , k;,r-transverse wave numbers in WL and ML, b2,3 -thickness of ML, n=1,3,5 ..., When the cutoff regime for E?: -waves take place, the transverse wave numbers k;iY + 0, and hence,