In this paper，we have given the definition of full-rank-primes. A prime P supposing that (p,10)=1 is defined as a full-rank-prime，if the length of the decimal repetend of 1/P is exactly P-l，roughly speaking. In the paper several interesting theorems about the decimal repetend of l/P when P is prime，especially a full-rank-prime，have been proved. For example，if P is a full-rank-prime，then the digit permutation of all K/P，1≤k≤p?1，has certain properties.