Honeycomb structure has been fabricated by brazing method using 0.1 wt%C and 1.0wt%C carbon steel core and STS304 stainless steel face sheet. Core shear strength ratio in W and L directions was 1:1.03 in 7 ㎜ cell size, whereas 1:1.45 in 4 ㎜ cell size. Flexural strength on face sheet was 166.4 MPa (0.1 wt%C, W direction), 171 .I MPa (0.1 wt%C, L direction), and 120.2 MPa (1.0 wt%C, W direction) in 7 ㎜ cell size. And in 4 ㎜ cell size specimen, it was 169.2 MPa (0.1 wt%C, W direction), 224.2 MPa (0.1 wt%C, L direction). This means that flexural strength of 0.1 wt%C core material was higher than that of 1.0 wt%C core material, which was contrary to expectation. SEM and EDS analysis represented that grain boundary diffusion had occurred in 0.1 wt%C core, but no grain boundary diffusion in 1.0 wt%C core. And corrugated surface of 0.1 wt%C core was flat, whereas that of 1.0 wt%C core was not flat. As a result, contact area between two 1.0 wt%C cores was much less than that of 0.1 wt% cores. It is thought to be main reason for lower flexural strength of 1.0 wt%C core.