Flexural toughness data were analyzed based on the assumption that S.F.R.C. load-deflection diagrams are similar to those of elasto-plastic materials. To prove that, toughness areas and toughness indices based on energies measured up to first crack point and energies measured to several chosen multiplies of first crack deflection were compared with those calculated values for elasto-plastic materials. The toughness indices based on plain concrete first crack deflections and first crack areas were higher than those based on S.F.R.C. first crack deflections and first crack areas. The first crack load defined as the load level at which load deflection diagram deviates from linearity can be misleading since in the case of highfiber coment the presence offibers causes the crack to extend very slowly and thus forced the load deflection diagram to maintain linearity beyond the actual first cracking load. Accordingly, the first crack areas ofS.F.R.C. were larger than those ofplain concrete due to the influence of fibers in increasing the first crack loads and first crack deflection. The average increase in first crack loads ofS. F. R. C. to those of plain concrete were 3, 11,8 and 20 percent for using 0.75%, 1.50% by volume of straight and deformed fibers respectivelyl6 J. Similar results were given by Snyder and Lankardf11J , ACI Commitee 544[1J and Mangatl121 .