Synchrotron-based　high-energy　X-ray　diffraction　is　used　to　explore　the　physical　origin　of　large　magnetostriction　in　DyCo2,　an　RT2　compound　(R　=　rare　earth,　T　=　Co,　Fe),　by　tracing　the　crystal　structural　change　as　a　function　of　temperature　and　magnetic　field.　When　the　DyCo2　compound　is　zero-field　cooled　down　below　the　Curie　temperature　TC,　the　high-temperature　cubic　lattice　is　distorted　into　a　tetragonal　structure,　associated　with　an　expansion　of　unit　cell　volume.　When　a　magnetic　field　is　applied　gradually　from　0　to　6　T　below　TC,　no　changes　in　the　peak　positions　for　tetragonal　(800)　T　and　(008)　T　peaks　are　observed,　whereas　their　relative　peak　intensities　are　gradually　changed.　The　intensity　changes　generated　during　magnetic　field　increasing　are　reversed　stepwise　with　decreasing　the　magnetic　field.　Our　experimental　results　suggest　that　the　large　magnetostriction　in　DyCo2　is　caused　by　the　crystallographic　domain-switch　mechanism　(or　rearrangement　of　tetragonal　domains).　The　diffraction　elastic　strain　is　not　detected　under　the　field　up　to　6　T.　The　present　investigations　provide　a　fundamental　understanding　of　the　mechanisms　of　the　large　magnetostriction　in　RT2　compounds　with　Laves　phases.　(C)　2017　Elsevier　B.V.　All　rights　reserved.