abT cell receptor (TCR) activation plays a crucial role for T cell function. However, the abTCR itself does not possess signaling domains. Instead, the TCR is non-covalently coupled to a conserved multi-subunit signaling apparatus, the CD3 complex, comprised of the CD3εγ, CD3εδ and CD3ζζ dimers. How antige ligation by the TCR triggers CD3 activation and what structural role the CD3 extracellular domains (ECDs) play in the assembled TCR-CD3 complex remain unclear. Here, we utilize two complementary structural approaches to gain insight into the overall organization of the TCR-CD3 complex. Small-angle X-ray scattering of the soluble TCR-CD3εδ complex reveal the CD3εδ ECDs to sit underneath the TCR α-chain.  The observed arrangement is consistent with electron microscopy (EM) images of the entire TCR-CD3 integral membrane complex, in which both the CD3εδ and CD3εγ subunits were situated underneath the TCR a-chain and TCR b-chain respectively. Interestingly, the TCR-CD3 transmembrane complex bound to peptide-Major Histocompatibility Complex (pMHC) is a dimer in which two TCRs project outward from a central core composed of the CD3 ECDs and the TCR and CD3 transmembrane domains. This arrangement suggests a potential ligand-dependent dimerization mechanism for T-cell receptor signaling. Collectively, our data advances our understanding of the molecular organization of TCR-CD3 complex, and provides a new conceptual framework for the TCR activation mechanism.