Mn55-x B45Co (x) bulk alloys with x ranging from 0 to 30 were fabricated by an arc melting technique. Structural properties were investigated by XRD and SEM analysis revealing that Co replaces Mn atoms as x amount increases in the alloy and samples contain two phases with Mn-rich or Co-rich regions. Magnetic properties were characterized by field dependence (M-H) and temperature dependence of magnetization (M-T) measurements using a vibrating sample magnetometer (VSM). Results showed that as x increases from 0 to 30, saturation magnetization decreases from 130 to 53 emu/g. In addition, an increase of the Co content leads a decrease of Curie temperature from 565 to 412 K, enabling the Curie temperature tuning by Co addition.