We demonstrated real-time control of micro/nanofiber waist diameter during fiber pulling process with ultrahigh accuracy and precision. A 785 nm CW laser was coupled into SMF-28e fiber to excite high-order modes. During the fiber-pulling process, the cutoffs of high-order modes contributed to sudden transmission intensity drops. By accurately measuring the time interval between two drops, we could precisely determine the time to stop the pulling process based on a target diameter. Our experimental results showed that both the accuracy and precision of diameter control were within 5 nm for an expected taper diameter ranging from 800 nm to 1300 nm.