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<jats:title>ABSTRACT</jats:title> <jats:p>We have carried out mapping observations of the entire L1551 molecular cloud with about 2 pc × 2 pc size in the <jats:sup>12</jats:sup>CO(1–0) line with the Nobeyama 45 m radio telescope at the high effective resolution of 22″ (corresponding to 0.017 pc at the distance of 160 pc), and analyzed the <jats:sup>12</jats:sup>CO data together with the <jats:sup>13</jats:sup>CO(1–0) and C<jats:sup>18</jats:sup>O(1–0) data from the Nobeyama Radio Observatory database. We derived the new non-thermal line width–size relations, σ<jats:sub>NT</jats:sub> ∝ <jats:italic>L</jats:italic> <jats:sup>γ</jats:sup>, for the three molecular lines, corrected for the effect of optical depth and the line-of-sight integration. To investigate the characteristic of the intrinsic turbulence, the effects of the outflows were removed. The derived relations are (σ<jats:sub>NT</jats:sub>/km s<jats:sup>−1</jats:sup>) = (0.18 ± 0.010)(<jats:italic>L</jats:italic>/pc)<jats:sup>0.45 ± 0.095</jats:sup>, (0.20 ± 0.020)(<jats:italic>L</jats:italic>/pc)<jats:sup>0.48 ± 0.091</jats:sup>, and (0.22 ± 0.050) (<jats:italic>L</jats:italic>/pc)<jats:sup>0.54 ± 0.21</jats:sup> for the <jats:sup>12</jats:sup>CO, <jats:sup>13</jats:sup>CO, and C<jats:sup>18</jats:sup>O lines, respectively, suggesting that the line width–size relation of the turbulence very weakly depends on our observed molecular lines, i.e., the relation does not change between the density ranges of 10<jats:sup>2</jats:sup>–10<jats:sup>3</jats:sup> and 10<jats:sup>3</jats:sup>–10<jats:sup>4</jats:sup> cm<jats:sup>−3</jats:sup>. In addition, the relations indicate that incompressible turbulence is dominant at the scales smaller than 0.6 pc in L1551. The power spectrum indices converted from the relations, however, seem to be larger than that of the Kolmogorov spectrum for incompressible flow. The disagreement could be explained by the anisotropy in the turbulent velocity field in L1551, as expected in MHD turbulence. Actually, the autocorrelation functions of the centroid velocity fluctuations show larger correlation along the direction of the magnetic field measured for the whole Taurus cloud, which is consistent with the results of numerical simulations for incompressible MHD flow.</jats:p>