Multicomponent phospholipid membranes provide an ideal model to study the complex phase behavior of biological membranes. Giant unilamellar vesicles (GUV) formed by mixtures of two or more phospholipids have particular merit as model membranes because of their simplicity, operability, and ease of viewing phase separation and testing membrane mechanics. Until the research in this thesis, biochemistry and biophysical studies of phase separation in phospholipid membranes primarily addressed the influence of membrane composition on the transition temperatures and domain shapes. This thesis focuses on a commonly neglected variable - membrane tension, analogous to pressure in bulk materials, as an important parameter to the phase separation. By exploring the GUVs formed from mixed 1,2-dioleoyl-sn-glycero-3-phosphocholine (DOPC) and 1,2-dipalmitoyl-sn-glycero-3-phosphocholine (DPPC) membranes, the study in this thesis comprehensively examines the thermodynamic impact of tension on fluid-solid membrane phase transition and the nature of phase-separated domains. Quantitative experimental studies of the temperature dependence on the membrane tension and compositions in DOPC/DPPC GUVs were mapped into a 3 dimensional (temperature-tension-composition) phase diagram. Depending on the system’s position in this temperature-tension-composition space, giant unilamellar vesicles containing DOPC and DPPC exhibited, in addition to a fluid phase, two different types of solid phases – ripple and tilt, presenting as patchy hexagonal and stripe domains respectively. Addressing the mechanism of domain formation, the study in this thesis also quantitatively examined the nucleation and domain growth in DOPC/DPPC vesicles. The observed increase of domain density with increases in the cooling rate and DPPC composition was consistent to the classical nucleation theory. Additionally, the domain density was found to be somewhat reduced by increases in membrane tension, consistent with the impact of membrane tension on line tension of the domain perimeter. Finally, this thesis extends the study to hybrid vesicles containing phospholipid (DPPC) and copolymer (DC 5329). Extensive studies employing model vesicles revealed similar phase behavior and tension sensitivity of domain formation as DOPC/DPPC vesicles. The findings demonstrated that lamella-forming copolymer could be substituted for low melting phospholipid components in membranes to enhance a membrane’s mechanical properties at the same time retaining key behaviors.