Dielectric breakdown strength of poly (ethylene-co-butyl acrylate) (EBA) nanocomposites filled with spherical alumina particles was studied as a function of particle coating and relative air humidity. The nanocomposites were prepared from a mixture of EBA formulations (13 wt% butyl acrylate groups content) and alumina powder. The particles were either unmodified or surface-treated with aminopropyltriethoxy silane or octyltriethoxy silane. The filler content studied was 6 wt%. Every material formulation was thoroughly examined under SEM in order to verify homogeneous particle dispersion. Two different relative humidities of air were used for conditioning the samples prior to testing: 0 and 86 % RH. The dielectric breakdown strength measurements were performed with a DC ramp of 1.2 kV/s. Ten samples were tested for each material formulation. Results were compared to the pristine EBA samples. Applying Weibull analysis to the measured data indicated a slight increase in breakdown strength for the dry nanocomposites filled with amino-treated particles as compared to the reference material in dry conditions. As expected, conditioning materials in humid environment had a negative impact on breakdown strength. This effect was more pronounced for nanocomposite materials, while the reference unfilled material was affected the least.