Experimental analysis was carried out to examine the influence of carbon content on the stress-strain curve in the vicinity of yield point. Numerical analysis was then carried out to investigate the mechanism in the microstructure through which the influence occurred. Attention was focused upon the morphology of the carbide, and the homogenisation method and the elastic-plastic finite element method were used for numerical evaluation. Despite very primitive assumptions that the macroscopic stress-strain curve of extra low carbon steel applies to the ferrite in the microstructure and the carbide deforms only elastically, the numerical results have explained well the stress-strain curves obtained experimentally. Finally, TEM observations were carried out to check the validity of the result predicted by the homogenisation method. The TEM images show a concentration of dislocations in the ferrite existing between adjacent carbides lying parallel with the macroscopic loading direction. This result explains qualitatively the localisation of strain in the ferrite predicted by the homogenisation method. By using the numerical analysis inversely, it would be possible to design the morphology of the carbide so that the macroscopic mechanical behaviour of a carbon steel meets the required value in the vicinity of yield point.