The quantitative imaging of attenuation coefficients slope (ACS) has the potential to improve medical diagnostics. However, attempts to characterize ACS using pulse-echo data have been limited by the large statistical variations in the estimates. Previous studies demonstrated that it is possible to extend the trade-off between variance and spatial resolution of quantitative ultrasound, spectral-based parameters by the use of full angular (i.e., 360°) spatial compounding (FASC). In the present work, the use of FASC has been extended to the estimation of ACS and its performance has been experimentally evaluated using two physical phantoms. The ACSs of the background and inclusion regions were estimated using insertion loss measurements to be 0.41 and 0.75 dB/cm/MHz for Phantom #1, and 0.54 and 1.04 dB/cm/MHz for Phantom #2, respectively. Pulse-echo data were collected using a 7.5 MHz, f/4 transducer at 30 angles of view uniformly distributed between 0 and 360°. Single view ACS maps were generated using a spectral log difference method with 0.6 by 0.6 mm data blocks. The FASC images were constructed by assigning to a pixel the median of its corresponding estimates from all 30 angles of view. The reduction in the variance of the FASC estimates compared to the variance of estimates from a single view (i.e., variance averaged from the 30 single views) in the inclusion and background regions were 89.18% and 88.71% for Phantom #1 and 92.33% and 86.98% for Phantom #2. Moreover, in all the cases the estimation bias in the inclusion and background regions using FASC was lower than 9.0%. These results suggest that the variance of attenuation coefficient slope estimation can be significantly reduced without sacrificing spatial resolution by the use of full angular spatial compounding.