In the negative pressure system of pneumatic conveying, the authors previously proposed a highly-dense and therefore highly-efficient feeding of a powder, which was equipped with an injection pipe in the center of the suction nozzle. In this paper, we examined the effect of pipe diameter ratio of injection pipe to suction nozzle on the loading ratio and nozzle efficiency. As a result, it was found that the optimum pipe diameter ratio exists for a given condition. The particles behavior near the suction nozzle at a different pipe diameter ratio was identified through the particle flow visualization.