Natural　gas　combustion　experiments　were　conducted　in　a　lean　burn　CNG　engine　to　uncover　the　regularities　of　cyclic　combustion　variations　(CCVs)　in　different　operating　conditions.　The　effects　of　the　engine　operation　parameters　on　the　correlations　between　pressure　related　characteristic　parameters　of　CCVs　were　also　investigated.　In　the　study,　the　level　of　CCVs　was　indicated　by　coefficient　of　variation　of　the　indicated　mean　effective　pressure　(COVIMEP)　and　the　investigation　was　conducted　by　two　different　ignition　timing　setting　methods,　which　were　fixed　ignition　timing　method　and　the　minimum　COVIMEP　(MCV)　ignition　timing　method.　The　results　show　that　the　minimum　COVIMEP　appears　at　the　optimal　ignition　timing　while　other　operation　condition　parameters　were　fixed.　And　the　trends　of　COVIMEP　versus　equivalent　ratio,　throttle　opening　position　and　engine　speed　separately　vary　significantly　by　setting　ignition　timing　at　different　fixed　value,　but　the　trends　were　not　disturbed　as　MCV　ignition　timing　method　is　used.　Furthermore,　by　computing　the　correlation　coefficient,　two　pairs　of　parameters,　which　were　the　maximum　cylinder　pressure　(P-max)　versus　its　appearance　time　(theta(pmax))　and　p(max)　versus　the　maximum　rate　of　pressure　rise　((dp/d　phi)(max)),　showed　stable　and　strong　linear　correlations.　However,　other　two　pairs　of　parameters,　which　were　IMEP　versus　p(max)　and　IMEP　versus　(dp/d　phi)(max),　were　influenced　obviously　by　the　equivalence　ratio,　engine　speed　and　throttle　opening.　(C)　2012　Elsevier　Ltd.　All　rights　reserved.