The present invention relates to a method for producing zeta negative single digit carboxylated nanodiamond dispersion. The method comprises adjusting pH of zeta negative carboxylated nanodiamond suspension to at least 7, and subjecting the pH adjusted suspension to beads milling. The present invention further relates to zeta negative single digit carboxylated nanodiamond dispersion comprising zeta negative single digit carboxylated nanodiamond particles and a liquid medium, wherein zeta potential of the zeta negative single digit carboxylated nanodiamond dispersion is over −37 mV measured at pH over 7, zeta negative single digit carboxylated nanodiamond particle concentration in the dispersion is over 2 wt-% and D90 average primary particle size distribution of the zeta negative single digit carboxylated nanodiamond particles is from 2 nm to 12 nm.