This　paper　presents　information　on　a　portable　fall　detection　and　alerting　system　mainly　consisting　of　a　custom　vest　and　a　mobile　smart　phone.　A　wearable　motion　detection　sensor　integrated　with　tri-axial　accelerometer,　gyroscope　and　Bluetooth　is　built　into　a　custom　vest　worn　by　elderly.　The　vest　can　capture　the　reluctant　acceleration　and　angular　velocity　about　the　activities　of　daily　living　(ADLs)　of　elderly　in　real　time.　The　data　via　Bluetooth　is　then　sent　to　a　mobile　smart　phone　running　a　fall　detection　program　based　on　k-NN　algorithm.　When　a　fall　occurs　the　phone　can　alert　a　family　member　or　health　care　center　through　a　call　or　emergent　text　message　using　a　built　in　Global　Positioning　System.　The　experimental　results　show　that　the　system　discriminates　falls　from　ADLs　with　a　sensitivity　of　95%,　and　a　specificity　of　96.67%.　This　system　can　provide　remote　monitoring　and　timely　help　for　the　elderly.