Miscibility of polylactide (PLA) and polyhydroxybutyrate (PHB) is studied by the microsecond atomistic molecular-dynamics (MD) simulations for the first time. The model and the simulation protocol were confirmed through comparison of the glass transition temperature (T g ) with experimental data. It was established that PLA and PHB are miscible on the basis of the 2 Flory-Huggins theory. Analysis of the mobilities of PLA and PHB subchains revealed that the blends have two transitions to a glassy state at the length scale of a few Kuhn segments, which is in line with the predictions of the self-concentration model. At the same time at the larger length scale, a single transition to a glassy state was observed suggesting scale-dependence of PLA and PHB miscibility. This scale-dependence was confirmed through the evaluation of the interchain pair correlation functions.