We take the design and evaluation of eukaryotie algae-targeted 18S rDNA PCR primers as an example, to give a step-by-step illustration of how to design and assess group-targeted PCR primers using Primrose and other programs. Then we summarize the advantages of the design ＆ evaluation pipeline by comparing the amplification ef- ficiency of newly designed and previously reported 18S rDNA universal primers. Cautions are also emphasized on u- sing PCR-based strategies to assess community diversity.