Complementary DNA library of a dinoflagellate Pyrocystis lunula was constructed for the purpose of expression sequence tags analysis. The RNA isolated from this alga was about 20 μg g-1 net cells, and the band intensity ratio of 28 S/18 S in electrophoresis pattern was nearly 1 to 1. Different cDNA/vector molar ratios were exploited in the ligating reaction to be optimized. The clones produced by cDNA/vector molar ratio of 3.75 to 1 were desirable, most of whose inserts were longer than 300 bp. The recombinants insert length of the unfractionation cDNA library was largely shorter than 500 bp. However, in the fractionation library made from high molecule weight cDNA parts, over seventy percent of the recombinants contained inserts longer than 1 kb, some of which were even longer than 3 kb. Operating concerns were discussed at the end.