Based on the theory of Mueller matrix of optical devices with both polarization dependent loss (PDL) and birefringence, the definition of polarization loss (PDL) is analyzed. The newly formulated equations for PDL are generalized for input light with arbitrary degree of polarization (DOP). Theoretical derivation shows that the overall polarization states should contain all the polarization states with random DOP. The magnitude of PDL has connection with the input DOP. The direction of PDL vector is only decided by the Mueller matrix.