Method of sizing carbon fibers

A method of sizing carbon fibers comprising the steps of:

• processing said carbon fibers with a water-based emulsion of sizing agents comprising one or more ester compounds selected from Group A described below and non-ionic surfactant at weight ratio of 90/10-30/70, said sizing agents having average diameter of 0.01-0.5 .mu.m in said water-based emulsion;
• and
• applying said sizing agents to said carbon fibers at a rate of 0.1-5.0 weight % with respect to said carbon fibers;
• said Group A consisting of ester compounds shown by Formula (1) below and ester compounds shown by Formula (2) below:
• ##STR2## and said surfactant being one or more selected from Group B consisting of polyalkoxylated aliphatic carboxylic acid esters of polyhydric alcohol, aliphatic carboxylic acid esters of polyoxyalkyleneglycol and polyoxyalkyleneglycol ethers of aliphatic alcohol, where R.sup.1 and R.sup.3 are each saturated aliphatic hydrocarbon group with 1-29 carbon atoms, unsaturated aliphatic hydrocarbon group with 15-21 carbon atoms or hydroxy substituted aliphatic hydrocarbon group with 15-21 carbon atoms;
• R.sup.2 is saturated aliphatic hydrocarbon group with 1-22 carbon atoms or unsaturated aliphatic hydrocarbon group with 14-22 carbon atoms;
• R.sup.4 is saturated alphatic hydrocarbon group with 1-22 carbon atoms, unsaturated aliphatic hydrocarbon group with 14-22 carbon atoms or alkylphenyl group having alkyl group with 4-12 carbon atoms;
• R.sup.5 is hydrogen atom, methyl group, ethyl group, phenyl group or phenoxymethyl group such that the total carbon atom number of R.sup.1 and R.sup.2 is 10 or greater than the total carbon atom number of R.sup.2 and R.sup.4 is 10 or greater;
• and n is an integer 1-20.