This work investigates the substitution of main-group element Al for Ge in the MnCoGe system to study the effect of magnetic phase transition and the magnetocaloric effect of the compounds. The Room temperature X-ray diffraction indicates clearly that most of the reflections can be identified with the orthorhombic structure (TiNiSi-type, space group Pnma) for MnCoGe and the hexagonal structure (Ni2In-type, space group P63/mmc) for MnCoAl. The substitution of Al for Ge in MnCoAl transmutes the crystal structure from TiNiSi-type to Ni2In-type structure. The magnetic measurement revealed the curie temperatures, Tc decreases from 340 K to 285 K with the substitution of Al with Ge. Besides, the Arrott plots (M2 against B/M) for the MnCoGe and MnCoAl, demonstrated characteristics of second-order transition for both samples. The calculated magnetic entropy change, -ΔSM values shows no difference for increasing and decreasing fields, which confirms that there is no hysteresis loss occurred near the magnetic transition. The -ΔSM maximal values is about 4.41 J·kg-1K-1 for MnCoGe and 3.73 J·kg-1K-1 for MnCoAl, for a field change of 0-5 T.
