Simple　Summary　Plutella　xylostell　is　a　worldwide　migratory　insect　pest　that　mainly　damages　cruciferous　vegetables.　In　this　study,　we　established　a　system　for　measuring　the　diel　locomotor　activities　and　used　it　to　evaluate　the　locomotor　circadian　patterns　of　P.　xylostella.　We　tested　the　locomotor　activities　of　P.　xylostella　adults　under　several　laboratory　settings.　We　found　that　both　the　males　and　females　showed　nocturnal　activity　under　a　light:dark　(LD)　cycle,　with　activity　peaking　very　early　after　lights　off　and　quickly　declining　after　lights　on.　Both　males　and　females　had　high　locomotor　activity　levels　in　constant　darkness　(DD)　but　weak　in　a　constant　light　condition　(LL).　In　addition,　circadian　patterns　analysis　showed　that　males　exhibit　much　better　rhythmic　characteristics　than　females,　especially　in　low　temperature　conditions.　Overall,　our　proposed　system　for　studying　the　locomotor　activities　in　P.　xylostella　is　reliable,　which　will　help　us　to　have　a　better　understanding　of　the　diel　activity　of　P.　xylostella　and　may　finally　be　helpful　in　the　development　of　an　effective　pest　management　strategy.　The　Diamondback　Moth　(Plutella　xylostella)　is　a　highly　destructive　lepidopteran　pest　of　cruciferous　crops.　However,　there　still　is　relatively　little　known　about　the　locomotor　activities　of　diel　and　the　circadian　patterns　in　P.　xylostella.　Here,　we　present　an　analysis　of　the　diel　locomotion　of　P.　xylostella　under　several　laboratory　settings.　We　established　a　system　for　measuring　the　individual　locomotor　activities　of　P.　xylostella　and　found　that　both　males　and　females　showed　a　nocturnal　pattern　of　activity　under　26　or　20　degrees　C　LD　conditions,　with　activity　peaking　immediately　after　lights　off　and　quickly　declining　after　lights　on.　In　addition,　we　showed　that　it　is　difficult　to　assess　the　free-running　circadian　rhythms　of　P.　xylostella　under　26　degrees　C　DD　conditions.　However,　we　found　that　males　showed　a　higher　power,　rhythm　index　(RI)　and　rhythmic　ratio　than　females　under　20　degrees　C　DD　conditions,　which　indicated　that　males　in　low-temperature　conditions　are　much　more　suitable　to　study　the　free-running　circadian　rhythms　of　P.　xylostella.　The　findings　of　this　study　will　help　us　to　have　a　better　understanding　of　the　diel　activity　of　P.　xylostella　and　may　provide　a　foundation　for　the　development　of　an　effective　pest　management　strategy.