In the present work, using the data base of the University of Michigan Radio Astronomy Observatory (UMRAO) at three radio frequencies (4.8 GHz, 8GHZ, and 14.5 GHz), we studied the polarization properties for 47 BL Lacertae objects: 38 radio selected BL Lac objects, 7 X-ray selected BL Lac objects, and two inter-middle objects (Mkn 421 and Mkn 501), and found that: (1) the polarizations at higher radio frequency is higher than those at lower frequency, (2) the variability of polarization at higher radio frequency is higher than those at lower frequency, (3) the polarization is correlated with the radio spectral index, (4) the polarization is correlated with the core-dominance parameter for those objects with known core-dominance parameters, suggesting that relativistic beaming could explain the polarization characteristic of BL Lacs.

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