The standard Bulgarian license plate consists of a white surface, using black font, containing a one- or two-letter province code, four numerals, and a final two-letter code, called the "series". They thus take the form X(X) NNNN YY, where X is the province code, and Y is the series. Since 1992, only letters (rather, glyphs) that are common to both the Cyrillic and the Latin alphabets are used on Bulgarian plates.

On the left hand side of the plate is the blue vertical "European strip", which contains the flag of Europe (or, for older-registered cars, flag of Bulgaria), below which is the country code for Bulgaria: BG.

More Info: en.wikipedia.org