A method for the determination of crosslink density for collagen fibers was proposed. The number of interchain crosslinkages in whale ligament and rat-tail tendon was estimated by applying an usual rubber elasticity theory. Collagen fibers swollen in a solution composed of equal volumes of 8M LiBr aqueous solution and diethylene glycol monoalkyl ether showed a typical rubber elasticity. The energy components to total retractive forces were similar in order in magnitude for crosslinked natural rubbers, namely, below 0.3. It was found that no intermolecular crosslinkage occurs between tropocollagen molecules in tendon from a 2-month-old rat, while there are about 12 crosslinking sites per molecule in tendon from a 10-month-old rat and 15 sites in whale ligament. The number, type, and crosslinking sites in the tendon crosslinked with 1,3-bis(vinylsulfonyl)-2-propanol is also discussed.