Using the aircraft observation data of the stratocumulus during the Routine AAF [Atmospheric Radiation Measurement (ARM) Aerial Facility] Clouds with Low Optical Water Depths (CLOWD) Optical Radiative Observations (RACORO) field campaign in April and May 2009, two methods are used to estimate the indirect aerosol effects: using the values defined by cloud drop concentration (AIEn) and by effective radius (AIEs). AIEn is almost always larger than AIEs, especially at moderate liquid water content. Theoretical derivation shows that the deviation between AIEn and AIEs should be related to the effect of aerosol on the dispersion of cloud droplet spectrum, e.g., the dispersion effect. When AIEn adds the dispersion effect, the value is very close to AIEs, confirming the theoretical expectations. The contribution of the dispersion effect to the aerosol indirect effect is mainly the offset effect, which is the largest at moderate liquid water content, reaching about 37% when liquid water content is 0.24 g/m3. The research results enhance the theoretical understanding of the aerosol-cloud interaction and will be helpful to improve the accurate assessment of the aerosol indirect effect in models and observations.