Strain　glass　transition　is　a　unique　nanoscale　displacive　transition　with　local　symmetry　breaking　while　maintaining　the　macroscopic　symmetry　or　average　structure　unchanged.　It　usually　occurs　in　the　＂nonmartensitic＂　composition　range　of　a　martensitic　system.　So　far,　only　indirect　evidence　exists　for　such　a　transition,　essentially　from　macroscopic　measurements　and　low-resolution　transmission　electron　microscopy　observations,　and　there　is　a　lack　of　direct　evidence　for　the　speculated　local　symmetry　breaking　and　the　sluggish　nature　of　the　glass　transition.　In　this　Letter　we　report　in　situ　high-resolution　transmission　electron　microscopy　observations　on　a　Ti50(Pd41Cr9)　strain　glass　alloy　and　direct　evidence　for　these　key　issues.　Our　results　show　that　at　temperatures　well　above　the　strain　glass　transition　temperature　(Tg),　the　lattice　is　essentially　an　undistorted　B2　structure.　With　approaching　Tg,　the　local　symmetry　breaking　gradually　occurs　with　the　formation　and　growth　of　nanomartensite　clusters　with　a　combined　stacking　period　of　three　and　four　plane　intervals,　but　the　average　structure　measured　by　x-ray　diffraction　remains　B2.　These　nanomartensite　clusters　become　finally　frozen　below　Tg.　Our　results　provide　not　only　a　microscopic　basis　for　the　macroscopic　properties　of　strain　glass,　but　also　new　insights　into　a　range　of　possible　applications　of　this　unique　class　of　materials.