Six equilibrium samples of (1−x)ZnSnAs2+xMnAs (x=0, 0.1, … 0.5mol.%) and one quenched sample of 0.8ZnSnAs2+0.2MnAs were synthesized. The samples were studied with X-ray, DTA, MFM and SEM. The system ZnSnAs2–MnAs has eutectic point with coordinates 45±2mol.% MnAs and 744±10°C. The eutectic compound contains isolated, practically round shape inclusions of MnAs in the host ZnSnAs2. Based on the earlier investigations, we predict negative magnetoresistance effect in this semiconductor–ferromagnet compound with nanogranular structures. The lattice parameters of ZnSnAs2 without manganese arsenide and ZnSnAs2+MnAs are similar, meaning MnAs does not influence the lattice parameters. Therefore one can make conclusion that solubility of the MnAs in ZnSnAs2 is low. The lattice parameters are a=5.8509Å, c=11.7135Å for tetragonal and a=5.8558Å for cubic symmetry. The density of the ZnSnAs2 calculated from the HR-XRD is 5.5314g/cm3 for tetragonal and 5.5230g/cm3 for cubic structure.