Clinical reference material of erythrocyte sedimentation rate (ESR) can ensure the accuracy and comparability of ESR in time and space to realize its effective traceability. Reference method of International Council for Standardization in Haematology (ICSH) is used to define the ESR value. This paper establishes a complete and rational assessment scheme for uncertainty of ESR reference material. Firstly the main source that affects the uncertainty of reference material is analyzed, then the uncertainty component introduced by stability, homogeneity and value definition process of the reference material is calculated respectively, and finally the uncertainty of reference material is calculated synthetically.