Abstract: The liquid scintillation counting system recently constructed for absolute activity measurement was introduced, and it uses TDCR method to measure activity of β nuclide. The system is equipped with three PMTs connected to a de-focusing voltage divider base developed by ourselves. After varying the efficiency by de-focusing of the PM tube, the activity of the ³H source was determined. The analysis of uncertainty and discussion of measurement result were given. The activity determined with TDCR is in agreement with certified value within the range of 0.9%. The combined uncertainty of results is 0.6% (k=1).