Abstract: According to the special feature that two-stage cluster sampling has the same efficiency as the systematic sampling or simple random sampling when there are no differences among clusters,the term of error expansion factor is presented in this paper. Besides the correlation coefficient for critical state,when efficiency coefficient equals to l,is formulated,which provide important reference of implementing cluster sampling design and sampling efficiency assessment.In addition,the relationship between efficiency of two-stage cluster sampling and distance between plots in a cluster is further studied with an example demonstrated in detail.Finally,the efficiency assessment of two-stage cluster sampling in forest inventory is also discussed.