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A series of single-phased RCo2B2 (R = Pr, Nd and Gd) compounds were prepared by an arc-melting method. The structure, magnetic properties and magnetiocaloric effect (MCE) were studied by the magnetization and heat capacity measurement. A paramagnetic to ferromagnetic (PM-FM) transition was observed and the Curie temperature TC is 18 and 27 K for PrCo2B2 and NdCo2B2, respectively. However, a paramagnetic to antiferromagnetic (PM-AFM) magnetic transition was observed ~15 K under low fields (H ≤ 0.2 T), whereas a field-induced metamagnetic transition from AFM to FM has been observed with the application of higher magnetic field (H ≥ 0.4 T) for GdCo2B2. A large/giant reversible MCE was observed around its magnetic transition temperature for RCo2B2 systems. For a field change of 5 T, the maximum values of magnetic entropy change (- ΔSM max) reach 8.1, 7.1 and 17.1 J kg-1 K-1 for R = Pr, Nd and Gd, respectively. The magnetic transition, origin of large MCE and its potential application in RCo2B2 systems were discussed.