Resolution in crosshole seismic tomography has been studied in this paper by means of ray density, ray orthogonality and point-spread function. Numerical experiments indicate that the resolution is better in the vertical direction than in the horizontal, and the high velocity anomalous body is different from the low velocity anomalous body in resolution and shapes of tomography. For the high velocity anomalous body, the spatial resolution of the waveform velocity tomogram in horizontal is about 1/2 of the wavelength, and that in vertical about 3/10 of the wavelength. For the low velocity anomalous body, the spatial resolution of the waveform velocity tomogram in horizontal is about 2/5 of the wavelength, and that in vertical about 1/5 of the wavelength. The spatial resolution of the traveltime velocity tomogram in horizontal is about three tomographic elements width, and that in horizontal about two tomographic elements width. When the ratio between the borehole depth and well offset is equal to 2.0, the spatial resolution of the velocity tomogram is the best.