  We derive the axis ratio distribution of X-ray clusters using the XMM-Newton
catalogue (Snowden et al. 2008). By fitting the contour lines of the X-ray
image by ellipses, we confirm the X-ray distribution is well approximated by
the elliptic distribution with a constant axis ratio and direction. We
construct a simple model describing the axis ratio of the X-ray gas assuming
the hydrostatic equilibrium embedded in the triaxial dark matter halo model
proposed by Jing & Suto (2002) and the hydrostatic equilibrium. We find that
the observed probability density function of the axis ratio is consistent with
this model prediction.
