An　electrochemical　immunosensor　for　microcystin-(leucine-arginine)　based　on　magnetic　bionanoparticles　and　iron(II,　III)　oxide　was　developed.　The　bionanoparticles　were　prepared　by　cross-linking　antibodies　of　microcystin-(leucine-arginine)　and　amino-functionalized　magnetic　iron(II,　III)　oxide　nanoparticles　with　glutaraldehyde,　followed　by　immobilization　on　the　surface　of　a　magnetic　electrode.　The　immunosensor　was　based　on　the　model　of　direct　competition,　as　microcystin-(leucine-arginine)　and　horseradish　peroxidase-conjugated　microcystin-(leucine-arginine)　competitively　combined　with　immobilizing　antibodies.　The　peak　current　of　differential　pulse　voltammetry　(DPV)　decreased　with　an　increase　of　microcystin-(　leucine-arginine)　concentration　after　antigen-antibody　reaction.　When　the　background　current　was　stabilized,　the　anodic　peak　current　response　and　change　in　peak　response　were　recorded.　Under　the　optimized　conditions,　the　change　in　response　was　proportional　to　the　microcystin-(leucine-arginine)　concentration　between　0.010　and　100　mu　g/L　with　a　limit　of　detection　equal　to　0.009　mu　g/L.　Amperometry　was　adopted　to　determine　microcystin-(leucine-arginine);　the　linear　dynamic　range　was　0.10　to　100　mu　g/L　with　a　detection　limit　of　0.08　mu　g/L.　The　method　was　successfully　applied　in　the　determination　of　microcystin-(leucine-arginine)　in　river　water　and　the　recoveries　were　between　90.2%　and　110.5%.