  Evolution of Fermi surface (FS) states of
NdFeAs$_{1-x}$P$_x$O$_{0.9}$F$_{0.1}$ single crystals with As/P substitution
has been investigated. The critical temperature $T_{\rm c}$ and the power law
exponent ($n$) of temperature-dependent resistivity ($\rho(T) = \rho_0 + AT^n$)
show a clear correlation above $x=$0.2, suggesting that $T_{\rm c}$ is enhanced
with increasing bosonic fluctuation in the same type of FS state. Around
$x=$0.2, all the transport properties show anomalies, indicating that
$x$$\sim$0.2 is the critical composition of drastic FS change. The angle
resolved photoemission spectroscopy has more directly revealed the distinct
change of FS around $x=$0.2, that one hole FS disappears at Brillouin zone
center and the other FS with propeller like shape appears at zone corner with
decreasing $x$. These results are indicative of the existence of two types of
FS state with different nesting conditions that are related with two $T_{\rm
c}$-rising mechanisms in this system.
