Optical coherence tomography (OCT) has been analyzed theoretically as an imaging system upon the weak object approximation. It has been proved that the OCT system behaves as a linear shift invariance system and can be described precisely by a coherent transfer function (CTF). The derived longitude CTF has shown that the system performs like a low pass filter with a FWHM Δ k=2 Δ λ/λ 2 . High spatial frequency structure is detected because the spectrum of the object shifted a distance of 1/λ . This research will provide a theoretical basis to quantitatively analyzed the measuring results of the unknown structure and to increase further the spatial resolution.