  In this paper, the compiled long-term optical and infrared measurements of
some blazars are used to analyze the variation properties and the optical data
are used to search for periodicity evidence in the lightcurve by means of the
Jurkevich technique and the discrete correlation function (DCF) method.
Following periods are found: 4.52-year for 3C 66A; 1.56 and 2.95 years for AO
0235+164;
  14.4, 18.6 years for PKS 0735+178; 17.85 and 24.7 years for PKS 0754+100;
5.53 and 11.75 for OJ 287. 4.45, and 6.89 years for PKS 1215; 9 and 14.84 years
for PKS 1219+285;
  2.0, 13.5 and 22.5 for 3C273; 7.1 year for 3C279;
  6.07 for PKS 1308+326; 3.0 and 16.5 years for PKS 1418+546;
  2.0 and 9.35 years for PKS 1514-241; 18.18 for PKS 1807+698;
  4.16 and 7.0 for 2155-304; 14 and 20 years for BL Lacertae. Some explanations
have been discussed.
