A sample including 935 blazars with 415 Flat Spectrum Radio Quasars (FSRQs) and 520 BL Lac objects (BL Lacs) (233 HBLs (High synchrotron peak frequency BL Lacs), 144 IBLs (Intermediate synchrotron peak frequency BL Lacs), and 143 LBLs (Low synchrotron peak frequency BL Lacs)) is selected in Fermi 3FGL catalogue. The correlations between two flux densities of the radio 1.4GHz and gamma-ray at 0.1, 0.3, 1, 3, and 10GeV are investigated for the samples of all blazars, FSRQs, BL Lacs, and subclasses of HBLs and LBLs, respectively. Results show that there are strong positive correlations between the two flux densities, with the correlation coefficients r = 0.48--0.81 and the chance probabilities p all less than 10^-4; The correlation coefficient has different trends with the change of gamma-ray emission frequency for different blazar samples, and the average correlation coefficients for all samples at the five respective bands decrease with the increase of gamma-ray frequency. Therefore, with the increase of gamma-ray frequency, the dominant mechanism of gamma-ray radiation of blazar is changing, and at the same frequency the dominant mechanism is different for different samples. The gamma-ray emission of HBLs may be mainly dominated by the synchrotron self-Compton, while the other mechanisms are responsible for the gamma-ray emissions of LBLs. The origin of gamma-ray in FSRQs is more complex than that of BL Lacs.