  We have conducted a systematic survey for the X-ray properties of millisecond
pulsars (MSPs). Currently, there are 47 MSPs with confirmed X-ray detections.
We have also placed the upper limits for the X-ray emission from the other 36
MSPs by using the archival data. We have normalized their X-ray luminosities
$L_{x}$ and their effective photon indices $\Gamma$ into a homogeneous data
set, which enable us to carry out a detailed statistical analysis. Based on our
censored sample, we report a relation of
$L_{x}\simeq10^{31.05}\left(\dot{E}/10^{35}\right)^{1.31}$ erg/s (2-10 keV) for
the MSPs. The inferred X-ray conversion efficiency is found to be lower than
previously reported estimate that could be affected by selection bias. $L_{x}$
also correlates/anti-correlates with the magnetic field strength at the light
cylinder $B_{LC}$/characteristic age $\tau$. On the other hand, there is no
correlation between $L_{x}$ and their surface magnetic field strength $B_{s}$.
We have further divided the sample into four classes: (i) black-widows, (ii)
redbacks, (iii) isolated MSPs and (iv) other MSP binaries, and compare the
properties among them. We noted that while the rotational parameters and the
orbital periods of redbacks and black-widow are similar, $L_{x}$ of redbacks
are significantly higher than those of black-widows in the 2-10 keV band. Also
the $\Gamma$ of redbacks are apparently smaller than those of black-widows,
which indicates the X-ray emission of redbacks are harder than that of
black-widows. This can be explained by the different contribution of
intrabinary shocks in the X-ray emission of these two classes.
