  New measurement by CELSIUS-WASA Collaboration on the pp->pn pi+ reaction
reveals clear evidence for the presence of the Roper resonance N*(1440) which
has been ignored in previous theoretical calculations. In this article, based
on an effective Lagrangian approach and available knowledge on the Roper
resonance, we investigate the role of the Roper resonance for the pp->pn pi+
reaction. It is found that the contribution from the Roper resonance N*(1440)
becomes significant for kinetic energy above 1.1 GeV, consistent with the new
experimental observation. The t-channel sigma-meson exchange is dominant for
the production of the Roper resonance.
