  We assume that the noncommutativity starts to be visible continuously from a
scale $\Lambda_{NC}$. According to this assumption, a two-loop effective action
is derived for noncommutative $\phi^{4}$ and $\phi^{3}$ theories from a
Wilsonian point of view. We show that these effective theories are free of
UV/IR mixing phenomena. We also investigate the positivity constraint on
coefficients of higher dimension operators present in the effective theory.
This constraint makes the low energy theory to be UV completion of a full
theory. Finally, we discuss noncommutativity and extra dimensions. In our
effective theories formulated on noncommutative extra dimensions, if the
campactification scale $\Lambda_{c}$ is less than the scale $\Lambda_{NC}$, the
theory will not suffer from UV/IR mixing.
