Cellulose　fiber　(CF)　paper　is　a　low-cost,　sustainable,　and　flexible　substrate,　which　has　gained　increasing　interest　recently.　Before　practical　usage,　the　functionalization　of　the　pristine　CF　paper　is　indispensable　to　meet　requirements　of　specific　applications.　Different　from　conventional　surface　modification　or　physical　mixing　methods,　we　report　in　situ　growth　of　ultralong　hydroxyapatite　nanowires　(HAPNWs)　with　lengths　larger　than　10　μm　on　the　CF　paper.　HAPNWs　are　radially　aligned　on　the　surface　of　CFs,　creating　a　micro/nanoscale　hierarchical　structure.　By　means　of　the　excellent　ion　exchange　ability　of　HAP　and　the　hierarchical　structure,　the　functions　of　the　CF　paper　can　be　easily　customized.　As　a　proof-of-concept,　we　demonstrate　two　kinds　of　functional　CF　paper:　(1)　the　photoluminescent　CF　paper　by　doping　Eu3+　and　Tb3+　ions　into　the　crystal　lattice　of　HAPNWs　and　(2)　the　superhydrophobic　CF　paper　by　coating　poly(dimethylsiloxane)　on　the　HAPNW　hierarchical　structure,　which　can　be　applied　for　self-cleaning　and　oil/water　separation.　It　is　expected　that　an　in　situ　growth　of　ultralong　HAPNWs　will　provide　an　instructive　guideline　for　designing　a　CF　paper　with　specific　functions.　©