To evaluate the role of non-metallic element (Si and C) on the microstructural evolution and mechanical properties of Fe_2.5CoNiCu high-entropy alloys (HEAs), two series of Fe_2.5CoNiCuSi_x (x=0.1-0.3, molar fraction) and Fe_2.5CoNiCuC_x (x=0.15-0.4, molar fraction) HEAs were prepared by vacuum induction melting. The results show that the amounts of C and Si additions significantly stimulate the transformation of BCC/FCC dual phases into a single FCC structure, and the microstructural evolution appears to be more strongly influenced by the C addition. The Fe_2.5CoNiCuSi_0.2 HEA presents high hardness (HV 439.5) and tensile strength (868 MPa), which is attributed to the solution strengthening and grain refinement effect. Both the hardness and strength of Fe_2.5CoNiCuC_x HEAs increase with the addition of C, due to the formation of carbides and solution strengthening. This demonstrates that adding non-metallic element (C or Si) is a viable approach for enhancing the strength and hardness of HEAs.