Abstract: The electronic structure and magnetism of Co_1-xMn_x alloys was studied by MS-Xα method. It is found that there are negative exchange couplings between atoms in Co_1-xMn_x alloys. The strength of negative exchange coupling increases with increasing of Mn atoms, and hence decreases the ferro-magnetism of bcc Co_1-xMn_x alloys. A good agreement is reached between the calculation and experimental results.