We experimentally determined the physico-chemical properties of seminal plasma as well as the sperm cryopreservation techniques of masou salmon, Oncorhynchus masou masou. Seminal plasma contained $18{pm}1mmol/L$ potassium, $144{pm}4mmol/L$ sodium, $116{pm}3mmol/L$ chloride, $83.2{pm}3.1mg/L$ calcium, $14.8{pm}0.7mg/L$ magnesium, $45{pm}9mg/L$ glucose, and $1.0{pm}0.0g/L$ total protein. The osmolality and pH of seminal plasma were $287{pm}7;and;7.7{pm}0.1mmol/kg$, respectively, and the spermatocrit was $28{pm}2$. The rate of embryonic survival at the eyed-stage and the hatching rate were highest in 10% methanol with 300 mM glucose. Compared to DMSO or glycerol, methanol served as a better cryoprotectant of masou salmon sperm.