For most of proteins to be active, they need well-defined three-dimensional structures alone or in complex. Folding is a process through which newly synthesized proteins get to the native state. Protein folding inside cells is assisted by various chaperones and folding factors, and misfolded proteins are eliminated by the ubiquitin-proteasome degradation system to ensure high fidelity of protein expression. Under certain circumstances, misfolded proteins escape the degradation process, yielding to deposit of protein aggregates such as loop-sheet polymer and amyloid fibril. Diseases characterized by insoluble deposits of proteins have been recognized for long time and are grouped as conformational diseases. Study of protein folding mechanism is required for better understanding of the molecular pathway of such conformational diseases.