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.
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