Diamond/SiC = 75/25~50/50 vol% composites were fabricated utilizing liquid-phase reaction sintering during hot isostatic pressing (HIP) at 1723 K under 196 MPa for 7.2 × 10³ s from mixtures of four kinds of diamond (average particle size P_s of 69, 60, 9.2, and 3.5 μmϕ) and Si (P_s: 0.6 μmϕ) powders. After the starting powders were mixed for 1.8 × 10³ s in ethanol, the mixtures were dried and compacted uniaxially (50 MPa) and isostatically (245 MPa). They were preheated at 1223 K for 7.2 × 10³ s in a vacuum (Process (I)) or solidified using pulsed electric-current pressure sintering (PECPS) at 1623 K and 50 MPa for 0.6 × 10³ s under reduced pressure (Process (II)). The compacts prepared via Process (I) and Process (II) were densified by Pyrex glass capsule HIPing (1723 K/7.2 × 10³ s/196 MPa/Ar). High relative density (around 93%) was achieved at the composition of diamond/SiC = 55/45 vol% using bimodal diamond powders, i.e., an average P_s of around 50 μmϕ and Process (II). In order to improve Vickers hardness (H_v), a small amount of B₄C (P_s: 0.5 μmϕ) powders was added to [diamond (69:3.5 μmϕ = 7:3)]/SiC = 55/45 vol% composite using Process (II) at 1623 K followed by HIP. Both the relative density and H_v values increased substantially, from 89 to 95.0% and from 38.7 to 51.3 GPa at 10 vol% B₄C addition, respectively.