A transient expression system for a unicellular marine green alga, Chlorella sp. MACC/C95, was developed using a reporter GUS gene coded for by plasmid pBI121. The results demonstrated a high transformation efficiency could be achieved by using electroporation to deliver DNA into intact cells and the CaMV35S promoter to drive the foreign gene expression in Chlorella sp. MACC/C95. The use of a carrier DNA coupled with osmosis treatment improved the transformation efficiency, while linearization of the plasmid had minor effects. Investigation of the effects of DNA concentration and growth phases of Chlorella sp. MACC/C95 on transformation efficiency indicated that the highest level of transient expression was observed when $6{mu}g;mL^{-1}$ of plasmid DNA and cells $2{sim}6$ days old were used.