Abstract: An oil cooling and filtering system of a transmission is described which comprises first and second oil passages formed in a given portion of a housing of the transmission. The given portion has an outer surface exposed to the outside of the housing. The first oil passage is fed with a compressed lubricating oil from an oil compressing means and the second oil passage is led to moving parts of the transmission which are to be lubricated. An oil cooler is connected to the outer surface of the given portion. The oil cooler includes a water flow passage and an oil flow passage which are isolated from each other. The oil flow passage has an inlet opening connected to the first oil passage and an outlet opening connected to the second oil passage. An oil filter is installed in at least one one of the first and second oil passages for filtering the lubricating oil flowing in the oil passage. With this arrangement, the oil cooler can be compact in size and simple in construction.