[en] Materials of the R_n+1Co_3n5B_2n (R=lanthanides; n=l, 2, 3, infinity) family are derived from the RCo₅ structure (P6/mmm space group), (n=0), by gradual substitution of B for Co on one of the Co sites, until complete substitution is achieved on that site for RCo₃B₂ (n=co). In the past, these materials have attracted a lot of interest because of their potential use as permanent magnets. Magnetic structure and properties of the TbCo₃B₂ compounds were previously studied in our group.This paper will show that in this system, Tb³⁺ serves as a good example for a 4f electron shell in a singlet ground state, which is formed under the influence of crystalline electric field. Nevertheless this system orders magnetically at T∼30 K where the resulted ground state magnetic structure at 1.5 K is ferrimagnetic, in which ordered Co and Tb moments are aligned in an anti-parallel formation, perpendicular to the unique axis