In this study, we utilized and modified ISO 14855-2, an industrial composting test, as a basis for conducting biodegradation tests on the poly(l-lactic acid) (PLLA)/amorphous polyhydroxyalkanoate (aPHA) blends under different conditions. Based on the obtained results, our objective was to investigate the reasons behind the biodegradability of PLLA under home composting conditions when blended with other materials. The compatibility of aPHA with PLLA in the blend interferes with the crystallization of PLLA via microphase separation, ultimately resulting in a dominantly amorphous PLLA structure. Additionally, the microphase separation of the two polymers created microfluidic channels by aPHA, which increased the surface area accessible to microorganisms in the compost, facilitating the biodegradation of both polymers. Furthermore, low-molecular-weight acids generated from aPHA contribute to the autocatalytic effect, further promoting biodegradation. Therefore, blending PLLA with aPHA has the potential to improve the biodegradability of PLLA and widen its range of applications.