Multi-wall nanotubes (MWCNTs) were added to $PEO-LiClO_4$ solid electrolytes in order to assess their enhancement of ionic conductivity. $PEO-LiClO_4-SiO_2$ composite electrolytes were also manufactured for comparison. In spite of the electrical conductivity of MWCNTs, $PEO-LiClO_4-MWCNT$ composites can be sucessfully used as solid electrolytes, provided that additive carbon nanotubes have discontinous forms and are homogeneously distributed within the $PEO-LiClO_4$ matrix. Shortening and uniform dispersion of MWCNT were identified as crucial factors for the performance of $PEO-LiClO_4-MWCNT$ composite electrolytes. Compared with the $PEO-LiClO_4-SiO_2$ composite, the addition of MWCNT to $PEO-LiClO_4$ resulted in greater enhancement in ionic conductivity. This might be attributable to enhanced amorphization by local modification of PEO chains from crystalline to disordered arrangements.