Substituent effect on the lifetimes of a series of substituted naphthalenes (Np) in the higher triplet excited state (T-n) was studied with transient absorption measurements using the two-color two-laser flash photolysis technique. Lifetimes of Np(T-n) in cyclohexane solution were determined from the triplet energy transfer quenching by carbon tetrachloride to be 0.98-63 ps. The different lifetimes of Np(L) were explained by the energy gap law for the internal conversion from Np(T-n) to Np(T-1), indicating that Np(T-n) quenched by carbon tetrachloride is assigned to Np(T-2) with the longest lifetime among Np(T-2). The lifetime of Np(T-n) was correlative with the Hammett sigma(p) constant. Electronic characters of substituents showed a more significant influence on the energy of the T-2 state than that of the T-1 state.