The aim of this study was to investigate rubber toughening of resol type phenol-formaldehyde resin. For this purpose, phenolic resin was first modified by acrylonitrile butadiene rubber alone, and then by using nitrite rubber together with 3-aminopropyltriethoxysilane. Test specimens were prepared by mixing and casting of liquid phenolic resin in three groups. In the first one, neat phenolic resin specimens were produced. In the second group, phenolic resin was modified with 0.5, 1, 2, and 3 wt% nitrile rubber, while in the last group modification was carried out by using 0.5 wt% nitrite rubber together with 1, 2, and 4 wt% amino silane (with respect to nitrite rubber). All specimens were heat cured in an oven.