The biobased carbon contents of rubber and rubber products were determined by the measurements of 14C concentration with an Accelerator mass spectrometer (AMS). The biobased mass contents were calculated from the measured values of AMS and the composition ratio, which was obtained by separation analysis for each component of solvent extract, rubbers and carbon black which included inorganic fillers. Eleven model vulcanized rubbers were prepared and tested to confirm the validity of the test method. The results highly correlated with the calculated value from the formulation. High repeatability and reproducibility have been obtained from the Round-robin test in nine laboratories in four countries. Optimization studies of AMS measurement conditions for rubber compounds and determination of the correction factor for biobased carbon contents from the value of percent modern carbon for the test result were also studied.