The development and creation of measuring instrument transformers and their testing equipment in improving accuracy and performance since the end of fifties is described. They include the development in error compensation for instrument transformers, high accuracy instrument transformers with a current booster or voltage booster, current comparators and two-stage instrument transformers, the comparator-type calibrators and its testing instruments, digital calibrators, etc. Future work in this field, will involve the development of instrument transformers with small secondary current and voltage, application of computers and microprocessors in instrument transformers and their testing equipment, etc.