An　electrochemical　DNA　biosensor　was　developed　for　detecting　a　gene　sequence　related　to　blooming　genera　of　Cyanobacteria,　Microcystis　spp.　In　this　sensor,　gold　nanoparticles　(Au　NPs)　were　electrodeposited　on　a　gold　electrode　(GE)　surface　to　enhance　the　amount　of　DNA　immobilization.　Silver　was　anchored　on　Au　NPs　to　increase　the　electrochemical　signal　of　hybridization　because　metallic　films　deposited　as　a　continuous　layer　on　a　solid　electrode　can　greatly　enhance　the　rate　of　electron　transfer.　In　the　differential　peak　voltammograms,　the　peak　current　of　the　methylene　blue　(MB)　hybridization　indicator　was　about　10　times　that　of　direct　detection.　Therefore,　lower　detection　limits　and　a　wider　linear　range　for　the　biosensor　were　obtained.　Based　on　silver　enhancement,　the　change　in　the　reduction　peak　current　before　and　after　hybridization　(Delta　i(p))　of　MB　was　linearly　related　to　the　concentration　of　the　target　DNA　sequence　in　the　range　of　3.0　x　10(-12)　to　1.2　x　10(-10)　mol　L-1　with　a　detection　limit　of　1.6　x　10(-12)　mol　L-1.　In　addition,　the　biosensor　had　good　selectivity,　and　the　base　mismatch　sequences　from　specific　species　could　be　distinguished.