The outgoing longwave radiation (OLR) bias due to the neglect of cloud horizontal inhomogeneities has been examined in this study. It is argued that this OLR bias is most significant for semi‐transparent cirrus clouds that are located in the cold upper troposphere. Using two cirrus cases observed from cloud radar, it is found that the OLR biases due to the plane‐parallel homogeneous assumption are ∼14 W m−2. These biases are largely caused by the horizontal variation of cloud optical depth. It is also shown that in general the OLR biases are strongly dependent on the cloud height and mean and standard deviation of cloud optical depth. We have demonstrated that the gamma‐weighted radiative transfer scheme, which is efficient for GCM applications, can be used to account for the effect of cloud horizontal inhomogeneity on the infrared fluxes.