This thesis reports the studies of the structure and mechanism of peptide-induced membrane pores by antimicrobial peptide alamethicin and by a peptide named Baxalpha5, which is derived from Bax protein. Alamethicin is one of best known antimicrobial peptides, which are ubiquitous throughout the biological world. Bax-alpha5 peptide is the pore-forming domain of apoptosis regulator protein Bax, which activates pore formation on outer mitochondrial membrane to release cytochrome c to initiate programmed cell death. Both peptides as well as many other pore-forming peptides, induce pores in membrane, however the structure and mechanism of the pore formation were unknown. By utilizing grazing angle x-ray diffraction, I was able to reconstruct the electron density profile of the membrane pores induced by both peptides. The fully hydrated multiple bilayers of peptide-lipid mixture on solid substrate were prepared in the condition that pores were present, as established previously by neutron in-plane scattering and oriented circular dichroism. At dehydrated conditions, the inter bilayer distance of the sample shortened and the interactions between bilayers caused the membrane pores to become long-ranged correlated and formed a periodically ordered lattice of rhombohedral symmetry, so that x-ray diffraction can be applied. To help solving the phase problem of diffraction, a brominated lipid was used and multi-wavelength anomalous diffraction was performed below the bromine K-edge. The reconstructed electron density profiles unambiguously revealed that the alamethicin-induced membrane pore is of barrel-stave type, while the Bax-alpha5 induced pore is of lipidic toroidal (wormhole) type. The underlying mechanism of pore formation was resolved by observing the time-dependent process of pore formation in vesicles exposed to Bax-alpha5 solutions, as well as the membrane thinning experiment. This demonstrated that Bax-alpha5 exhibited the same sigmoidal concentration dependence as alamethicin: below a threshold concentration, peptide only binds to membrane surface, and thins the membrane; when the concentration exceeds a critical value, pores are formed. The structure and mechanism of peptide-induced membrane pores provide insight onto how alpha-pore-forming proteins and peptides interact with membrane. The results also suggest that formation of lipidic pores is the major mechanism for alpha-pore-forming proteins.