Organic thin-film transistors (OTFTs) with a buffer layer sandwiched between source/drain electrodes and organic semiconductor are demonstrated. An intrinsic molecular semiconductor, Lutetium bisphthalocyanine (LuPc2), is used as the buffer layer due to its high carrier density (1016 cm3). Compared with conventional OTFTs, the introduction of the buffer layer leads to on-state current increases from 700 nA to 2.5 μA, field-effect mobility increases from 0.7×10-2 to 1.58×10-2 cm2/V s, and threshold voltage downshifts from -21 to -11 V for the linear region. The on/off current ratio is improved to a level of 104. Mechanisms of performance improvement are attributed to include the difference of the Fermi level and interface dipolar between LuPc2 and Au. Our results demonstrate that it is an effective method to improve linear region characteristics by using a molecular semiconductor as the buffer layer.