The error in the domain averaged cloud optical thickness retrieved from satellite-based imagers is investigated using a cloud field generated by a cloud model and a 3D radiative transfer model. The cloud field used in the simulation is a relatively uniform (retrieved shape parameter of a gamma distribution averaged over all simulated viewing and solar zenith angles is 18) and nearly isotropic stratocumulus field. The retrieved cloud cover with a 1 km pixel resolution is 100%. The domain averaged optical thickness error is separated into two terms, the error caused by the extinction coefficient variability outside a pixel (external variability) and inside a pixel (internal variability). For the cloud field used in this study, the external variability term increases with solar zenith angle and the sign changes from negative to positive while the internal variability term is generally negative and becomes more negative as the solar zenith angle increases. At a small solar zenith angle, therefore, both terms are negative but the error partially cancels at a large solar zenith angle. When the solar zenith angle is less than 30o, both terms are small; the error in the viewing zenith angle and domain averaged cloud optical thickness derived from the relative azimuth angle smaller than 150o is less than 10%. However, if the optical thickness is derived from nadir view only for overhead sun, the domain averaged optical thickness is underestimated by more than 10%. When the solar zenith angle increases to 60o, the internal variability term exceeds 10% especially viewed from the forward direction but the domain and viewing zenith angle averaged optical thickness error can be less than 10% in the backward direction. When the solar zenith angle is 70o, both terms are greater than 10%. The shape parameter of a gamma distribution derived from retrieved optical thicknesses increases with the viewing zenith angle but decreases with solar zenith angle. Based on this simulation and Moderate Resolution Imaging Spectroradiometer (MODIS) viewing geometry and solar zenith angle at the sampling time over the eastern Pacific the error in the domain averaged retrieved optical thickness of uniform stratocumulus over eastern Pacific is less than10% in March and September.