In the present study, systematic rheometric measurements are carried out to explore quasi-sinusoidal responses in the nonlinear regime of carbon black-filled monodispersed polyisoprene melts with molecular weights ranging from 3 to 390 kg/mol. Experimental results show that these quasi-sinusoidal responses emerge from a sharp transition as the molecular weight of the matrix, Mn, approaches and passes through a characteristic molecular weight M-c*. Below M-c*, the system typically shows the classic nonlinearity, where storage modulus G' decreases as strain amplitude gamma(0) increases and the resulting stress waveforms are distorted from sinusoidal waves. Above M-c*, the system displays anomalous nonlinearity, where the stress responses at any given strain remain surprisingly sinusoidal regardless the drop of modulus G'. In this manner, the system actually exhibits characteristic linear-nonlinear dichotomy in the rheological responses in the nonlinear regime. The critical point M-c* is found to be a few times of the entanglement molecular weight Me. Here, we introduce the linear-nonlinear-dichotomy transformation diagram for the particle-filled polymer system under oscillatory shears, delineating the boundaries between states of linear response, nonlinear response, and linear-nonlinear dichotomy response. (C) 2017 The Society of Rheology.