A water-dilutable emulsion lubricant for machining, exhibiting improved dispersibility and emulsion stability, by emulsifying an oil system, composed of a base oil and an effective amount of fine particles of boron nitride of a crystalline turbostratic structure, dispersed in the base oil, in a water system, using an emulsifier. The lubricant is free from heavy metals, e.g., molybdenum disulfide and useful for machining difficult-to-cut materials such as Ti alloys and Inconel. The lubricant is effective even at a diluted state comprising 0.001% up to 0.1% by weight of crystalline t-BN.