Studies of magnetic and thermal properties have been carried out in yt
terbium monopnictides YbX (X = N, P, As and Sb). Combining the new exp
erimental results with other measurements, many thermal and magnetic p
roperties were found to change regularly from YbAs to YbP and to YbN.
This suggests the strength of magnetic exchange interaction increases
and the strength of Kondo interaction (or the effect of magnetic polar
on) decreases from YbAs to YbP and to YbN. YbSb is a very exceptional
case.