Abstract: Mean relative molecular mass (M) of alkyldimethylbenzyl-quaternaryammonium salt was determined and calculated by ion-pair HPLC with peak area normalization. The sample solution was separated on C18 silicon gel bonding chromatographic column (4.6 mm×250 mm, 5 μm) with mixed solution of methanol and water in the ratio 95 to 5 by volume (containing 10 mmol·L-1 pentyl sulfonic acid) as mobile phase, and detected at 262 nm by diode array detector (DAD). The proposed method was applied to the determination of M of the sample 1227, value of M found was 344.5. Analytical result by the present method was compared with that by GC, as verified by F-test and t-test, no significant difference was found.