Standardized sampling area is the most commonly used way while comparing the diversities of different communities in vegetation science. The key reason is, the larger the sampling area is, the more species are in the samples. However, samples from different communities with the same area doesn’t mean that each sample has the same representativeness of the community. It leads to bias by violating the replication principles. The comparison of samples at different standardized sampling area might get different results unless to use the diversity indices ignoring the rare species. If the study purposes do not want to ignore the influence of rare species in calculating the diversity, standardizing samples by their completeness is more reasonable than by their sampling size. This research used simulation data to show the bias caused by standardizing samples with the same sampling size. The results indicate that sampling area is one, but not the best one of standardizing the completeness among samples. Change the sampling scheme or use the statistical estimation of diversities can standardize the sampled completeness and make an unbias comparison of diversites in different communities.