About 20 Planetary Nebulae (PNe) belonging to the spiral galaxy M33 have been analyzed with the photoionization code CLOUDY 94.00 (Ferland et al. 1998) and with the Ionization Correction Factors method. Central star and nebular parameters have been determined. The chemical abundances of He/H, O/H, N/H, Ar/H, and S/H have been measured. Chemical abundance relationships have been investigated; in particular, a correlation between N/H and N/O similar to the Galactic one and an anti-correlation between O/H and N/O have been found. No gradient of metals across the disk of M~33 is seen at the level of our accuracy in abundance determinations. The present result shows that oxygen and helium abundances (with lower accuracy also nitrogen, argon and sulphur) can be actually estimated from the brightest PNe of a galaxy, even if the electron temperature cannot be directly measured. We also found that oxygen abundance is quite independent of the absolute magnitude of the PN and consequently the brightest PNe are in this respect representative of all PNe population. This represents an important tool to measure the metallicity of galaxies by the time of the formation of PNe progenitors.