The effect of heat treatment on the resistivity and structure of electroless Co-P plating films was investigated. The results obtained are as follows. 1) The two types of ammonia alkaline solutions were employed as plating baths. The deposition rate of these baths was affected by bath pH and the phosphorus content in deposited films varied in the range of 1.1-4.3mass%. 2) The resistivity of deposited films was affected by phosphorus content of films and it increased with increasing phosphorus content value. Upon heating the as-plated films, the resistivity was observed to change drastically. These films ware classified into two groups according to thermal behavior of their resistivities. 3) The structural change depending on heat treatment varied considerably with the state of as-plated films. In the films with P>4%, the amorphous structure was transformed into a structure consisting of aggregate mixture of hcp Co and Co₂P at 550-650K. While in the films with P<4%, remarkable structural change can not be recognized under 650K and gradual grain growth was observed over 750K. 4) The mechanism responsible for the change of resistivity seems to be described by two different processes. One is the annihilation of grain boundaries which decreases the resistivity, the other process is the formation of Co₂P which tends to increase the resistivity.