The effects of pressure anisotropy on the tearing mode in a current sheet are studied based on a linear compressible MHD code. It is found that in the presence of a small pressure anisotropy (P⊥>P∥), tearing mode dominates the unstable modes except that the growth rates are enhanced, which is consistent with the recent results; for a large pressure anisotropy the growth rates of unstable modes become even larger, and a mixed mirror-tearing configuration is formed for large kx (wave number along current sheet) implying that mirror instability is responsible for the enhancement of growth rate and modification of the tearing mode. In the presence of a large pressure anisotropy, a new oscillating tearing mode is observed when the wave number kx is small. Furthermore, a large y component of the magnetic field is found to be able to cancel the effect of pressure anisotropy, with the resultant mode being the same as that of isotropic tearing instability.