[Background] Reprogramming of somatic cells into pluripotent stem cells have opened the new-era in regenerative medicine. Viral transduction of defined factors successfully achieved pluripotency from mouse and human somatic cells. However, in the generation process of induced pluripotent stem (iPS) cells, genetic integration of certain factors may cause mutagenesis or tumorigenicity, which hampers further application for cell therapy. [Methods and Results] We investigated that the transient cellular delivery of mouse ES cell-derived proteins enables dedifferentiation of mouse somatic cells into pluripotent stem cells without genetic manipulation. We found that transient delivery of ES cell-derived proteins using reversible permeabilisation can activate endogenous transcription factors related pluripotency. Dedifferentiated somatic cells are morphologically indistinguishable from the authentic ES cells. In the process, the methylation and histone modification status in the promoter regions of pluripotency-related genes are converted from somatic to ES-equivalent statues. Molecular biologically, the patterns of global gene expression, genomewide ChIP sequencing and proteomics using Itracq are very similar to those of protein donor ES cells. Functionally, dedifferentiated cells form embryoid body (EB) structures in suspension culture. On 7 days of re-differentiation, cells contract spontaneously and express cardiac troponin T and 慣-sarcomeric actinin, indicating the properties of cardiomyocytes. And also the certain portions of re-differentiated cells express ectoderm, mesoderm and endoderm proteins. Furthermore, dedifferentiated cells possess in vivo differentiation and developmental potentials including well-differentiated teratoma formation, contribution to chimeras and germline transmission. [Conclusions] These results demonstrate that transient delivery of ES cell-derived proteins can induce dedifferentiation of adult somatic cells into pluripotent stem cells. Our results provide an alternative and safe strategy for dedifferentiation or reprogramming of adult somatic cells and may facilitate tailored- or patient-specific pluripotent stem cell-derived cell therapy.