Open-pore porous materials can be produced by the capsule-free Hot Isostatic Pressing (HIPing) process. In this paper, sintering mechanisms, features and applications of capsule-free HIPed open-porous materials are discussed. During sintering powder compacts by capsule-free HIPing process, high pressure gas penetrates inside of the green bodies, and forms pores. There are two mechanisms to explain this phenomenon. Surface-diffusion is enhanced during capsule-free HIPing, because high frequency gas-collision with pore surfaces, forms grown necks between raw powder particles, and enhances pore formation. Another possible mechanism is the matter of spatial compatibility of gas atoms and sintered material. For the shrinkage of pores, i. e., for the densification of the green bodies, the high pressure gas, or the high density gas must be removed from inside of pores. The higher density requires the higher energy to remove high density gas. At the end of the sintering process, the gas must be removed from pores to ambient pressure to make those pores to be open pores. The above mentioned sintering mechanisms provide higher open porosity, more grown necks, smoother pore surfaces and narrower pore size distribution in capsule-free HIPed materials compared with those in conventionally sintered porous materials. As a consequence, high Young's modulus, high fracture strength, high fluid permeability are obtained by the capsule-free HIP process. The porous materials fabricated by the capsule-free HIPing can be applied for filters, grinding wheels, electrochemical electrodes and others with better performance than conventional products.