We　have　designed　a　novel　isothermal　cascade　signal-amplification　strategy　for　ultrasensitive　colorimetric　determination　of　nucleic　acids.　It　is　based　on　double-cycling　amplification　with　formation　of　DNAzyme　via　a　polymerase-induced　strand-displacement　reaction　and　nicking　endonuclease-assisted　recycling.　The　assay　makes　use　of　a　hairpin　DNA,　a　short　primer,　KF-polymerase,　and　nicking　endonuclease.　The　presence　of　a　target　DNA　triggers　the　strand-displacement　and　polymerization　reaction　with　the　formation　of　numerous　DNAzyme　molecules.　Upon　addition　of　H2O2　to　the　resulting　mixture,　the　H2O2　reacts　with　2,2＇-azino-bis　(3-ethylbenzothiozoline)-6-sulfonate　to　form　a　colored　product　in　the　aid　of　DNAzyme,　which　is　quantified　by　photometry　at　415　nm.　Under　optimal　conditions,　the　assay　allows　target　DNA　to　be　determined　at　concentration　as　low　as　0.6　aM.