We estimate the average Co magnetic moment and account for the moment variation by a model in Yn+1Co3n+5B2n ( n = 0, 1, 2, 3 and infinity), Prm+nCo5m+3nB2n ( m = 2, n = 1 and m = 2, n = 3), Ndm+nCo5m+3nB2n ( m = 3, n = 2) and Y2Co14B compounds. And we obtain the magnetic properties of hypothetical Ym+nCo5m+3nB2n ( n = 1, m = 2, 3, 4 and 5) compounds. (c) 2005 Springer Science + Business Media, Inc.