  The low-temperature (LT) superstructure of $\alpha'$-NaV$_2$O$_5$ was
determined by synchrotron radiation x-ray diffraction. Below the phase
transition temperature associated with atomic displacement and charge ordering
at 34K, we observed the Bragg peak splittings, which evidence that the LT
structure is monoclinic. It was determined that the LT structure is
$(a-b)\times 2b \times 4c$ with the space group $A112$ where $a, b$ and $c$
represent the high temperature orthorhombic unit cell. The valence estimation
of V ions according to the bond valence sum method shows that the V sites are
clearly separated into two groups of V$^{4+}$ and V$^{5+}$ with a $zigzag$
charge ordering pattern. This LT structure is consistent with resonant x-ray
and NMR measurements, and strikingly contrasts to the LT structure previously
reported, which includes V$^{4.5+}$ sites.
