Abstract: Carbon nanotubes (CNTs) were functionalized by acid. CNTs treated with or without acid treatment were tested by FT-IR and SEM, which was demonstrated that the acid treatment shorten the CNTs, the impurities were removed from the CNTs, and several groups such carboxyl groups and hydroxy groups were inserted in CNTs. Phenolic resin modified by acid CNTs was synthetized by the solution polymerization and as-received products were then analyzed by FT-IR. The impact strength and shear strength of phenolic resin modified by acid CNTs presented the trend which was increased firstly, and then decreased with CNTs content increased. Their corresponding maximums were about 271kJ•m-2、72MPa, and increased 41kJ•m-2、17MPa respectively when the content of CNTs was 1.5 wt%.