The possibility and applicability of thermal recycling of waste PET-resin have been investigated in the open atmospheric operation by supplying PET-resin powder as an auxiliary or additional fuel to the proposed twin-fuelled burner. In the preceding experiments, flow characteristics and burning processes of PET-particles in the proposed burner were investigated using PIV/PTV measurements. Appearances of flying PET-particles were optically observed and analyzed by using the doublepulsed laser, and the particle diameter was evaluated as an equivalent diameter. According to the results, some of the key factors for reducing the unburnt rate of PET-particles were clarified. In this paper, by taking these factors into consideration, an experimental attempt to reduce unburnt particle emission in the PET-resin powder combustion is carried out by varying the diameter of PET-powder supply pipe. The results specify that the increase in the dispersion degree of PET-particles reduces certainly the unburnt rate of PET-powder.