The absorption features of DNA nucleobases cytosine and thymine were measured by terahertz time-domain spectroscopy (THz-TDS) from 0.1 to 3.5 THz. Our experimental results clearly show that these important biomolecules exhibit distinctive absorption features in THz region. To the best of our knowledge, the subtle absorption peak of cytosine at 2.53 THz is reported for the first time. Moreover, geometry optimizations and lattice dynamic calculations on cytosine crystal were also performed with the pseudo-potential plane wave method of density functional theory by taking periodic boundary conditions into account. All measured terahertz absorption features of cytosine were assigned successfully and its absorption spectrum was reproduced according to our calculations. Furthermore, our results show that absorption features of cytosine below 3.5 THz arise from external modes in translation and rotation motions, which are dominated by the intermolecular hydrogen bonds.