  We investigate the impact of mergers on the mass estimation of galaxy
clusters using $N$-body + hydrodynamical simulation data. We estimate virial
mass from these data and compare it with real mass. When the smaller
subcluster's mass is larger than a quarter of that of the larger one, virial
mass can be larger than twice of the real mass. The results strongly depend on
the observational directions, because of anisotropic velocity distribution of
the member galaxies. We also make the X-ray surface brightness and
spectroscopic-like temperature maps from the simulation data. The mass profile
is estimated from these data on the assumption of hydrostatic equilibrium. In
general, mass estimation with X-ray data gives us better results than virial
mass estimation. The dependence upon observational directions is weaker than in
case of virial mass estimation. When the system is observed along the collision
axis, the projected mass tends to be underestimated. This fact should be noted
especially when the virial and/or X-ray mass are compared with gravitational
lensing results.
