The critical plasma spraying parameter (CPSP) is a significant factor to influence the quality of plasma-sprayed coatings. The aim of this work was to investigate the effects of the CPSP on microstructure and mechanical properties of nanostructured alumina-13 wt.% titania (n-AT13) coatings prepared by supersonic plasma spray (SPS). The microstructure, phase composition, porosity, micro-hardness, Young's modulus and fracture toughness of coatings were characterized and experimentally measured. The results revealed that the values of porosity, micro-hardness, Young's modulus and fracture toughness followed Weibull distribution and had wide ranges. The microstructure of the coating consisted of fully melted regions and partially melted regions, which resulted in a bimodal distribution characteristic of mechanical properties. With the increase of CPSP, there were two contradictory trends. The mean values and characteristic values of both porosity and fracture toughness decreased rapidly and then slowly to a local minimum, while values of both micro-hardness and Young's modulus were on the contrary trend. Moreover, the characteristic values of micro-hardness and Young's modulus were decreased functions of those of porosity, and the opposite tendency for the characteristic value of fracture toughness. (C) 2014 Elsevier B.V. All rights reserved.