By the use of drilling records taken in oil fields in Japan, the factors affecting penetration rate of a rotary drilling were analysed. Some of the results obtained are as follows: (1) In some fields there is a relationship between load on a bit and penetration rate both for vertical wells and directional wells drilled with a vertical deviation angle of less than 40 degree. (2) In other fields there is no special relationship between bit load and penetration rate. In these fields hydraulic factors must be considered to analyse the penetration rate. (3) When soft mudstone (its compressive strength: 135kg/cm2) at a depth of 3700 meter was drilled with heavy bit load, it gave a very low penetration rate because the pressure caused by a long column of mud fluid was very high.