  The disk surface density of the nearby spiral galaxy M33 is estimated
assuming that it is marginally stable against gravitational perturbations. For
this purpose we used the radial profile of line-of-sight velocity dispersion of
the disk planetary nebulae obtained by Ciardullo et al. (2004). The surface
density profile we obtained is characterized by the radial scalelength which is
close to the photometrical one and is in a good agreement with the rotation
curve of M33 and with the mass-to-light ratio corresponding to the observed
color indices. However at the galactocentric distance $r>7$ kpc the dynamical
overheating of the disk remains quite possible. A thickness of the stellar disk
of M33 should increase outwards. The dark halo mass exceeds the mass of the
disk at $r>$ 7 kpc. The obtained radial profile of the disk surface density and
the radial gradient of $O/H$ are used to calculate the effective oxygen yield
$Y_{eff}$ in the frame of the instantaneous recycling approximation. It is
shown that $Y_{eff}$ increases with radius which may indicate that the role of
accretion of metal-poor gas in the chemical evolution of interstellar medium
decreases outwards.
