Abstract: This article Presents the study of some mechanical tests (tensile,impact, fatigue, and cold embrittlement) for several types of widmanstatten structure of 40 carbon steel. It was emphatically considered on the factor of grain size during tests, and was proceeded by comparing the condition both with defferent classes of Widmanstatten structure at the same grain size and with the same class of Widmanstatten structure but at different, grain size. Test results revealed that within the experimental environment the characteristic of needle-shape ferrite of the simply Widmanstatten structure has no obvious effect to decrease the mechanical properties. This explains why we usually confused that the Widmanstatten structure has any injurious effect on the mechanical properties of steels and the reason is not due to characterestics of needle a-ferrite but is due to some causes as overheating. Therefore; it is unsuitable by roaghly considering that the Widmanstatten structure has injurious influence on the mechanical properties.