A synthetic coprisin analog peptide, 9-mer dimer CopA3 (CopA3) was designed based on a defensin-like peptide, Coprisin, isolated from the bacteria-immunized dung beetle Copris tripartitus. Here, CopA3 was investigated for its antimicrobial activity and cancer cell growth inhibition. CopA3 showed antimicrobial activities against various pathogenic bacteria and yeast fungus with MIC values in 2~32 μM ranges, and inhibited the cell viabilities of pancreatic and hepatocellular cancer cells, except MIAPaca2, Hep3B, and HepG2 cells, in a dose-dependent manner. The average IC50 values of CopA3 against pancreatic and hepatocellular cancer cells were 61.7 μM and 67.8 μM, respectively. The results indicate that CopA3 has potential in the treatments of pancreatic and hepatocellular cancers as well as microorganism infection disease.