To analyze genetic diversity and phylogeny of Gracilariaceae (Gracilariales

Rhodophyta). We compared ITS（including 5.8S rDNA） sequences of twenty three populations of five

Gracilaria

and

Gracilariopsis

species (Gracilariaceae

Rhodophyta)

with the ITS sequences of 16 Gracilariaceaen species downloaded from GenBank database. The result showed that the length of ITS sequences of Gracilariaceae varies from 893 to 1 508 bp. The interspecific genetic distance ranges from 0.041 to 0.600; the intraspecific genetic distance ranges from 0.000 to 0.012

the former is much higher than the latter. Two main lineages of Gracilariaceae were from:

Gracilaria

\

Hydropuntia

and

Gracilariopsis

/

Gracilariophila

. 5.8S sequences of Gracilariaceae has low variation among species and among populations

but the distinct sites were found among the genera and can be used on genus level or above. There are 9 mutant sites that can be used to distinguish the populations of

G. vermiculophylla

from China

Russia and USA.