The effects of internal and external mixings of black carbon (BC) (soot) in ice particles on the radiative properties of contrail cirrus are investigated using a simple ice plate model. The internal mixing state absorbs substantially more radiation as compared with its external mixing counterpart due to light absorption enhancement from all directions. The soot absorption effect is largely confined to wavelengths shorter than about 1.4 µm, beyond which ice absorption predominates. For an ice crystal size of 5 µm internally mixed with a soot particle of 0.1 µm radius in contrail cirrus with an optical depth of 0.5, the instantaneous radiative forcings at the top of and within the atmosphere are approximately 0.2 and 0.8 W/m2, respectively. In view of the likelihood of multiple inclusions of soot particles in contrail cirrus, these values are lower limits. Thus, a realistic assessment of the global and regional radiative forcings of contrail cirrus for climate studies must account for the radiative effect induced by soot mixing states associated with the microscopic formation of ice particles.