Highly　sensitive　detection　of　K-ras　gene　is　of　great　significance　in　biomedical　research　and　clinical　diagnosis.　Here,　we　developed　a　colorimetric　biosensing　system　for　the　detection　of　proto-oncogene　K-ras　based　on　enhanced　amplification　effect　of　DNA　molecular　machine,　where　dual　isothermal　circular　strand-displacement　amplification　(D-SDA)　occurs　on　two　arms　in　one-to-one　correspondence.　Specifically,　we　designed　a　primer-locked　hairpin　probe　(HP)　and　a　primer-contained　linear　polymerization　template　(PPT).　In　the　presence　of　target　gene,　HP　can　hybridize　with　PPT,　forming　a　DNA　molecular　machine　with　dual　functional　arms　(called　DFA-machine).　Each　of　the　two　probes　in　this　machine　is　able　to　be　extended　by　polymerase　on　its　counterpart　species.　Moreover,　with　the　help　of　nicking　endonuclease,　the　dual　isothermal　polymerization　is　converted　into　dual　circular　strand-displacement　amplification,　generating　a　large　amount　of　anti-hemin　aptamer-contained　products.　After　binding　to　hemins,　the　aptamer/hemin　duplex,　horseradish　peroxidase　(HRP)-mimicking　DNAzyme,　was　formed　and　catalyzed　the　oxidation　of　colorless　ABTS　by　H2O2,　producing　a　visible　green　color.　The　proposed　colorimetric　assay　exhibits　a　wide　linear　range　from　0.01　to　150 nM　with　a　low　detection　limit　of　10 pM.　More　interestingly,　the　mutations　existing　in　target　gene　are　easily　observed　by　the　naked　eye.　It　should　be　noted　that　this　colorimetric　system　was　proved　by　the　analysis　of　K-ras　gene　of　SW620　cell　lines.　The　simple　and　powerful　DFA-machine　is　expected　to　provide　promising　potential　in　the　sensitive　detection　of　biomarkers　for　cancer　diagnosis,　prognosis　and　therapy.　©　2016　Elsevier　B.V.