Angular distributions of brightness temperatures over the Amazon rain forest observed by the NOAA 16 Advanced Microwave Sounding Unit-A (AMSU-A) window channels are studied and simulated with a radiative transfer model. The model is based on a combination of the atmospheric radiative transfer model and a vegetation canopy model which treats the rain forest as a uniform layer with an effective canopy temperature. Since emission from the ground surface is heavily attenuated by the rain forest canopy, only radiations from the rain forest canopy and the atmospheric medium contribute to the measurements. The measured angular distributions of brightness temperatures at the four window channels 1--3 and 15 (with frequencies centered at 23.8, 31.4, 50.3, and 89 GHz, respectively), have relatively small angular dependence that is attributed to the atmospheric path length since the canopy radiation is angular-independent. Data used in this study are collected from April 2001 to October 2002 for 1 month in each season. Angular distributions of the data from each month show remarkably stable patterns which can be well simulated by the model developed in this study. For the four window channels, ascending (day time 2 PM in the Sun) and descending (nighttime 2 AM in dark) measurements differ by 3--8 K, depending on channels. The root-mean-square (RMS) differences between 2 years' angular distributions for all four channels and the three overlapping months (April, July, and October) are 0.82 K for the ascending passes, and 0.47 K for the descending passes, respectively. The stable pattern and small variation of angular distributions can be potentially useful for postlaunch calibration of future microwave instruments.