Particle　Image　Velocimetry　(PIV)　was　used　to　measure　the　flow　field　of　staggered　square　pin-fin　array　in　a　wide　rectangle　channel　(AR=4).　The　experiment　was　conducted　at　two　Reynolds　number,　10000　and　20000,　based　on　the　hydraulic　diameter　and　bulk　velocity　of　the　channel.　The　distribution　of　flow　field　properties　was　compared　with　that　of　Nu　to　analysis　the　key　flow　physics　driving　heat　transfer　enhancement　in　channel　with　square　pin　fin.　The　Nusselt　number　was　achieved　through　temperature　measurement　using　thermochromic　liquid　crystal　in　the　same　geometry　setup.　Results　were　compared　with　those　for　circular　pin　fin　to　study　the　effect　of　geometry　on　flow　physics　driving　heat　transfer　enhancement.　It　was　found　that　the　wake　length　of　square　pin　fin　is　longer　than　that　of　circular　pin　fin,　which　indicated　flow　around　square　pin　fin　requires　longer　distance　to　develop.　Compared　to　circular　pin　fin,　small　scale　disturbances　in　the　shear　layer　of　square　pin　fin　show　its　contribution　to　local　end　wall　heat　transfer　enhancement.　Large　motions　benefit　end　wall　heat　transfer　more　effectively　at　lower　Re.　Small　scale　unsteadiness　contributes　more　to　heat　transfer　augment　as　flow　develops　or　Reynolds　number　increases　while　large　scale　motions　get　weaker.　©　2021　by　ASME.