Accumulating evidence has demonstrated that up-regulation of the angiotensin (Ang)-converting enzyme (ACE)/AngII/AngII type 1 receptor (AT1R) axis aggravates pulmonary fibrosis. The recently discovered ACE2/Ang-(1-7)/Mas axis, which counteracts the activity of the ACE/AngII/AT1R axis, has been shown to protect against pulmonary fibrosis. However, the mechanisms by which ACE2 and Ang-(1-7) attenuate pulmonary fibrosis remain unclear. We hypothesized that up-regulation of the ACE2/Ang-(1-7)/Mas axis protects against bleomycin (BLM)-induced pulmonary fibrosis by inhibiting the mitogen-activated protein kinase (MAPK)/NF-κB pathway. In vivo, Ang-(1-7) was continuously infused into Wistar rats that had received BLM or AngII. In vitro, human fetal lung-1 cells were pretreated with compounds that block the activities of AT1R, Mas (A-779), and MAPKs before exposure to AngII or Ang-(1-7). The human fetal lung-1 cells were infected with lentivirus-mediated ACE2 before exposure to AngII. In vivo, Ang-(1-7) prevented BLM-induced lung fibrosis and AngII-induced lung inflammation by inhibiting the MAPK phosphorylation and NF-κB signaling cascades. However, exogenous Ang-(1-7) alone clearly promoted lung inflammation. In vitro, Ang-(1-7) and lentivirus-mediated ACE2 inhibited the AngII-induced MAPK/NF-κB pathway, thereby attenuating inflammation and α-collagen I production, which could be reversed by the Mas inhibitor, A-779. Ang-(1-7) inhibited AngII-induced lung fibroblast apoptotic resistance via inhibition of the MAPK/NF-κB pathway and activation of the BCL-2-associated X protein/caspase-dependent mitochondrial apoptotic pathway. Ang-(1-7) alone markedly stimulated extracellular signal-regulated protein kinase 1/2 phosphorylation and the NF-κB cascade. Up-regulation of the ACE2/Ang-(1-7)/Mas axis protected against pulmonary fibrosis by inhibiting the MAPK/NF-κB pathway. However, close attention should be paid to the proinflammatory effects of Ang-(1-7).