We study the metallicity properties of galaxies in the zCOSMOS sample in the range 0.2 < z < 0.8. At z < 0.46, where Hα and [N ii] are detected, we find the same dependence of metallicity on stellar mass and star formation rate, the fundamental metallicity relation (FMR), found by Mannucci et al. in Sloan Digital Sky Survey galaxies on a similar redshift range. We extend this relation to higher redshift, 0.49 < z < 0.8, where the R23 metallicity index can be measured in our data, finding no evidence for evolution, and a metallicity scatter around the relation of about 0.16 dex. This result confirms, with a much higher level of significance with respect to previous works, the absence of evolution of the FMR during the last-half of cosmic history.