White spot syndrome virus is currently the most devastating viral pathogen threatening the shrimp farming industry worldwide. Studying efficiency of Galaxaura elongata as red marine algae on that shrimp virus was the aim of this work. Cells culture from lymphoid organ of Penaeid semesulacatus were established and when confluent cell monolayer was formed, the cell were infected with 0.1ml virus of titer 106.4 TCID50/ml mixed with (5, 10, 20 µl) algal extract. Data cleared algal extract of G. elongata has antiviral effect to WSSV where it results in reduction of virus titer 1.3 logs at concentration 5 µl/well, 2.2 logs at concentration10 µl/well and the highest reduction was at concentration 20 µl/well which was (2.5 log10TCID50). Algal extract was reduced CPE of infected cells in the attachment assays by 55, 70 and 75% consecutively and in the trypsin inhibition assay a significant activity was determined for 10 µl extract concentration. So, it can be suggested that using of G. elongata as red algae has vitro effect against WSSV replication.