  For a Galois number field $K$, P\'olya-Ostrowski group of $K$ is the subgroup
of the ideal class group of $K$ generated by all strongly ambiguous ideal
classes. $K$ is called a P\'olya field, whenever its P\'olya group is trivial.
In this paper, using some results in \cite{Bennett, Zantema}, we give an
explicit relation between order of P\'olya groups and Hasse unit indices in
real biquadratic fields.
