In this paper, we analyze the relativistic Thomson scattering with few-cycle laser pulses. We conclude that scattering of high energy electrons with mono-cycle laser pulses is capable of producing attosecond pulses. The calculation shows that with a 1.5 cycle pulse at the wavelength of 1 μm, the normalized electric field intensity of 0.01,and the electron relativistic factor of 50, X-ray pulses of 0.2 as can be obtained. We also discussed the influence of the carrier-envelop phase offset and the initial phase φin of electrons starting to interact with the optical field on the radiation spectrum. The calculation results demonstrate that proper values of phase can result in single attosecond pulse generation, as well as the possibility of frequency tuning.