The investigations have been made on the effects exerted upon the operating conditions and yield of product by changing the properties and components of cracker feed stock, and the following results are obtained:(i) It is most desirable for the cracker feed to contain the oils of small contents of heavy metals dissolved, such as nickel, vanadium, iron, and copper.These desirable feed stocks are the vacuum distillates and the slack wax obtained from the solvent dewaxer. In general, the allowable limits of these metals dissolved in feed stock can not be determined clearly, because the contamination of catalyst by these metals is closely connected with its stack loss.(ii) The dissolved metals in feed stock are proportional to its carbon residue. It is possible to presume the metals contents in the stock by measuring its carbon residue.(iii) The yield of cracked gasoline is proportional to the aniline point of its feed stock. Therefore, the feed stock having high aniline points is recommended for the production of the gasoline with high yield.(iv) As the deasphalted oil contains more metals than the other distillates, it is desirable to use the feed stock containing less deasphalted oil or to use the highly refined deasphalted oil.(v) There is a linear relation between the contamination index and the nickel content in the stationary operation; therefore, it should be possible to indicate the contamination degree of the catalyst by nickel content in place of the contamination index.