An experimental study on a part of the Cr-Nb-Mo-Si quaternary phase diagram associated with the NbCr2 Laves phase is conducted. It is found that most of the Si added to 50Cr-30Mo-20Nb alloys is distributed in the NbCr2 Laves phase and that the amount of Mo in the Laves phase increases with increasing Si addition. The Laves phase in Cr-Mo-Nb alloys with higher Si addition is found to be more stable. Electronic structure calculations based on density functional theory are performed to examine the effects of Mo and Si co-doping on the formation enthalpy of NbCr2. It is found that the co-doping effectively increases the stability of the Laves phase. This result is consistent with the effect of the atomic size ratio D-A/D-B on the stability of the AB(2) Laves phase, where D-A and D-B are the average atomic diameters of atoms occupying the A and B sites of the AB(2) structure, respectively.