The　synchronicity　of　the　measurement　data　at　different　locations　of　a　structure　is　essential　to　structural　health　monitoring.　Sometimes,　however,　non-synchronicity　may　occur　in　them.　A　frequency　domain　approach　is　proposed　for　the　estimation　of　the　relative　time　lag　between　vibration　measurement　data.　Making　use　of　the　mapping　between　a　relative　time　lag　in　the　time　domain　and　a　phase　angle　shift　in　the　frequency　domain,　the　relative　time　lag　between　either　two　outputs　of　a　structure　is　determined　by　finding　a　mode　where　the　actual　phase　angle　between　them　is　ideally　zero　and　then　calculating　the　perturbed　phase　angle　that　corresponds　to　the　modal　frequency　of　the　selected　mode.　To　address　the　problem　of　non-uniqueness　of　solutions,　the　slope　of　the　phase　angle　curve　is　employed　to　identify　the　actual　relative　time　lag　from　a　pool　of　candidates.　Extensive　validations　have　been　carried　out　with　the　use　of　filed　measurement　data.　Asynchronous　acceleration　data　recorded　during　a　ship-collision　of　the　Jiangyin　Bridge　were　employed　to　examine　its　capability　to　fulfill　correct　identification　of　relative　time　lag.　Synchronous　acceleration　data　of　the　Canton　tower　under　ambient　excitations　were　exploited　to　test　its　capability　to　avoid　false-positive　identification　of　relative　time　lag.　The　proposed　frequency　domain　approach　achieved　a　satisfactory　performance　in　the　identification　of　relative　time　lag　for　both　asynchronous　and　synchronous　measurement　data.　(C)　2018　Elsevier　Ltd.　All　rights　reserved.