When the Wiener spectrum of a flat panel detector (FPD) is calculated, scan is carried out using a virtual slit. By changing the slit length of virtual slit, we investigated how the Wiener spectrum would be influenced. Consequently, the Wiener spectral value rose as the slit length was enlarged, and the Wiener spectral value was converged in 16-pixels slit length. The Wiener spectral value did not change in 16 pixels or more slit length. Therefore, the slit length of virtual slit using for the calculation of Wiener spectrum of FPD should just use 16-pixels slit length.