Recently, doping non-carbon atoms into graphene or carbon nanotube (CNT) has attracted considerable attention due to its effectiveness to change or tailor their electronic and magnetic properties as well as chemical reactivity. In this work, we present a density functional theory study of the recently synthesized phosphorus (P) doped graphene and CNT. Particular attention is paid to studying the effects of P-doping on the structural, electronic, and magnetic properties as well as chemical reactivity of graphene or CNT. The results show that P dopant drastically changes the geometrical structure of graphene or CNT, rendering P and its neighboring C atoms protrude from the sidewall of CNT and graphene. Moreover, P-doping induces localized electronic states into graphene and CNTs, thus modifying the electronic properties by producing n-type behavior. Meanwhile, due to P doping, the graphene and CNT exhibit magnetic nature with spin net moment of 1.02 and 0.99 mu(B), respectively. In order to evaluate the chemical reactivity of the two nanostructures, their interactions with several gas molecules, including NH3, H2O, O-2, NO2, and NO, are further calculated. Our results may be useful not only for deeply understanding the properties of CNTs and graphenes, but also for developing various novel nanodevices. Crown Copyright (C) 2013 Published by Elsevier B.V. All rights reserved.