In geochronology, the isotope dilution (ID) method has been developed as an analytical method that provides accurate and precise element concentration and isotope ratio. For the ideal application of spiked sample, the critical prerequisite is the accurate isotopic composition of spike, which is commonly challenging to determine accurately. In this study, we report the first independent measurement of lutetium (Lu) isotopic composition in a Lu spike by multi-collector inductively coupled plasma mass spectrometry (MC-ICP-MS) using an optimized regression mass bias correction model. The rhenium (Re) reference material (NIST SRM 3143), previously calibrated against the NRC Ir isotopic CRM IRIS-1, was selected as the primary calibrant to calibrate the Lu isotope ratio in Lu spike. The obtained ratio of "R" _"175/176" ^"Lu" was 0.19994 ± 0.00050 (u, k=1), which meets the required precision levels for the Lu-Hf isotopic system application. The accuracy of the Lu - Hf isochron age was improved by the application of the precisely calibrated Lu spike, and this Lu spike was successful applied for the determination of Lu concentration in geological reference material. The application of the optimized regression model was further broadened by determining the isotopic composition of the Lu spike.