Abstract: Nine species of microalgae were tested for ingestion and digestion of Trachycardium flavum larvae using optical microscope to choose an oppropriate diet. An experiment was conducted using 2 dah, 6 dah,10 dah, 14 dah and 18 dah larvae. Larvae were stocked in flasks and fed with 105 indmL-1 of each species individually. Larvae were fed for 1 h and then observed under the microscope to detect ingestion; larvae were then sieved and placed in flasks containing filtered seawater to analyze digestion for the microalgae ingested. Among the nine species, Chlorella vulgaris, Nannochloropsis oculata, Pavlova viridis, Dicrateria zhanjiangensis and Isochrysis galbana were ingested at any stage, while the other species were ingested with the larvae growth, but C.vulgaris and N.oculata were hardly digested; with increasing concentration, the microalgae will digested partially or not digested. Therefore,P.viridis, D.zhanjiangensis and I.galbana should be used for larvae rearing of T.flavum, feeding little and several times.