  We study the growth of black holes and stellar population in spheroids at
high redshift using several (sub)mm-loud QSO samples. Applying the same
criteria established in an earlier work, we find that, similar to IR QSOs at
low redshift, the far-infrared emission of these (sub)mm-loud QSOs mainly
originates from dust heated by starbursts. By combining low-z IR QSOs and
high-z (sub)mm-loud QSOs, we find a trend that the star formation rate
($\Mstardot$) increases with the accretion rate ($\Mdot$). We compare the
values of $\Mstardot/\Mdot$ for submm emitting galaxies (SMGs), far-infrared
ultraluminous/hyperluminous QSOs and typical QSOs, and construct a likely
evolution scenario for these objects. The (sub)mm-loud QSO transition phase has
both high $\Mdot$ and $\Mstardot$ and hence is important for establishing the
correlation between the masses of black holes and spheroids.
