Macrophages (Mphi) are prominent components of solid tumors and exhibit distinct phenotypes in different microenvironments. We have recently found that tumors can alter the normal developmental process of Mphi to trigger transient activation of monocytes, but the underlying regulatory mechanisms are incompletely understood. Here, we showed that the protein expression of transcription factor C/EBPbeta was markedly elevated in tumor-associated Mphi both in vitro and human tumors in situ. The expression of C/EBP protein correlated with cytokine production in tumor-activated monocytes. Moreover, we found that C/EBPbeta expression was regulated at the post-transcriptional level and correlated with sustained reduction of microRNA-155 (miR-155) in tumor-activated monocytes. Bioinformatic analysis revealed that C/EBPbeta is a potential target of miR-155 and luciferase assay confirmed that C/EBPbeta translation is suppressed by miR-155 through interaction with the 3'UTR of C/EBPbeta mRNA. Further analysis showed that induction of miR-155 suppressed C/EBPbeta protein expression as well as cytokine production in tumor-activated monocytes, an effect which could be mimicked by silencing of C/EBPbeta. These results indicate that tumor environment causes a sustained reduction of miR-155 in monocytes/Mphi, which in turn regulates the functional activities of monocytes/Mphi by releasing the translational inhibition of transcription factor C/EBPbeta.