Variability studies of blazars have identified in the past hints of quasi-periodic emission for different sources, although strong evidences are still lacking. The understanding of whether this emission is truly periodic or not can help to explain the structure of blazars and the emission mechanisms taking place in these objects. We present the results of the periodicity analysis in the optical band of the gamma-ray blazars 3C 66A and B2 1633+38 with light curves spanning more that a decade. Three well-known tools commonly applied in periodicity studies have been used: the Lomb-Scargle periodogram, the Z-transform Discrete Correlation Function and the Weighted Wavelet Z-transform. We have found significant evidences of long-term periodicity in these sources, with periods of several years. The study was complemented with the periodicity analysis of the gamma-ray light curves. The results are interpreted within the framework of blazar emission models.