Cloud optical thickness and the effective radius of cloud droplets were derived by a combined use of the precipitation radar and visible and infrared scanner onboard the Tropical Rainfall Measuring Mission satellite to study the effect of precipitation on the relationship between cloud optical thickness (τ) and the effective radius (re) on a global scale. Whether τ and re correlate positively or negatively is a key to finding evidence of the indirect effects of aerosols, but up to now this has been a subject of considerable debate. No clear correlation has been reported on a global scale because τ is affected by many factors, among which the effect of precipitation is one of the most uncertain. To study the effect of precipitation, we assessed the relationship between τ and re in terms of the critical radius of cloud droplets (rc), below which precipitation hardly forms. In our analysis, there was no clear correlation between τ and re in water clouds which is consistent with many previous studies. However, interesting features of the relationship were revealed that are related to rc. Cloud optical thickness is a maximum for clouds with re around the value of rc. On a global average, cloud optical thickness tends to increase with re (positive correlation) for re < rc and to decrease (negative correlation) for re > rc. A change in the sign of the relationship was clearly observed at re ∼ rc on a global scale. These features were observed typically for non-precipitating clouds and clouds with weak rain. The relationship is strongly affected by precipitation which should be carefully considered when using it to find evidence of the indirect effects of aerosols.