To examine the effects of aggregation-inducing motifs related to neurodegenerative diseases on amyloid formation of host protein, we prepared several chimera myoglobins, in which various aggregation-inducing motifs were inserted. The focused aggregation-inducing motifs included five (R5) or two (R2) oligopeptide repeats in yeast Sup35p, five octapeptide repeats (OPR) in the human prion protein, a nonamyloid beta component (NAC) in alpha-synuclein, and tandem repeats of 50 glutamines (Q50). Circular dichroism and infrared spectroscopies suggested that the OPR, R5, and Q50 motifs formed an antiparallel beta sheet as well as a random coil, whereas the R2 and NAC motifs mainly formed random coils. The OPR, R5, and Q50 mutants, but not the R2 and NAC mutants, readily formed the SDS-resistant aggregates under physiological condition, and electron microscopy revealed that the aggregates contained amyloid fibrils. The destabilization and increase in gyration radius of the OPR, R5, and Q50 mutants correlated with the tendency to form amyloid fibrils. A control mutant bearing a nonamyloidgenic sequence was also moderately destabilized but did not form amyloid fibrils. Therefore, we concluded that the OPR, R5, and Q50 motifs, even in a quite stable protein such as myoglobin, led the host protein to formation of amyloid fibrils under physiological condition.