Co and Co95TM5 (TM = Fe, Cr and Mn) nanoalloys were synthesized by thermal decomposition method of transition metals acetylacetonate in organic solvents. The X-ray diffraction (XRD) results confirm the presence of a competition between cubic and hexagonal phases of the Co metallic. For samples containing 5% of manganese or chromium, we have the presence a small amount of Co in a hexagonal crystal structure in contrast of the Fe-doped sample which present only the cubic phase. Transmission electron microscopy (TEM) image shows that the particles present average size of 5 nm with spherical-like shape. The samples containing 5% of Fe and Cr exhibit a maximum magnetization approximately two times more than the obtained for the pure Co and Mn-doped samples.