Influence of ultrafine carboxylic acrylonitrile butadiene rubber powder (XNBRP) on properties of natural rubber (NR) was investigated. Mechanical properties, thermal resistance, and oil resistance of XNBRP-filled NR were determined after various loadings of XNBRP were added. Results clearly show that the presence of XNBRP not only leads to increased compound viscosity but also causes deterioration of the mechanical properties, e.g., tensile strength, tear strength, abrasion resistance, rebound resilience, as well as compression set at elevated temperature. This is mainly attributed to the reduced cross-link density and the poor interfacial adhesion between NR and XNBRP However, both oil and thermal ageing resistances are found to improve continuously with increasing XNBRP loading. Such improvement is obvious when 20 phr of XNBRP is loaded. Although the rubber becomes softer after oil immersion, it is surprisingly found that the modulus at high strain of the swollen rubber is greater than that of the unswollen one. The network tightening effect is proposed to explain the results. (C) 2011 Wiley Periodicals, Inc. Adv Polym Techn 30: 183-190, 2011; View this article online at wileyonlinelibrary.com. DOI 10.1002/adv.20215