Network　covert　channels　can　be　used　to　transfer　secret　information　in　network　protocols.　For　a　kind　of　network　covert　channels　hidden　in　overt　sources　with　fixed　probability　distribution,　we　propose　a　modified　Pearson　chi-square　test　which　improves　detection　accuracy.　We　incorporated　adjacent　probability　values　and　divided　the　new　distribution　into　two　partitions　sensitive　to　different　hidden　sections.　In　addition,　given　an　overt　source　and　different　covert　parameters,　we　can　calculate　the　relation　between　covert　capacity　and　detection　probability.　Advantages　of　the　detection　method　are　verified　by　an　experiment　using　real　network　traffic　data.