Summary Effects of eccentric tools on acoustic LWD measurements are investigated in both time domain and frequency domain using stretched grid finite difference algorithm and finite element eigenvalue approach, respectively. Monopole, dipole and quadrupole LWD tools are discussed. The study shows that an eccentric LWD tool affects the acoustic field in two aspects: First, it generates multipole modes thus reduces signal to noise ratio of the desired mode. For instance, in quadrupole logging, strong Stoneley mode is observed when the tool is not centralized; Secondly, it changes dispersion behavior of each individual mode. Specifically, Stoneley mode becomes less dispersive and higher modes such as dipole and quadrupole split into two. Compressional arrival is still observable with eccentric monopole tool, but with lower energy and less coherence in semblance domain. Further investigation demonstrates that for practical purpose, even severe tool eccentricity has little effect on dipole or quadrupole logging with appropriate data processing.