The greatest common factor (GCF) of 9 and 27 is 9. One way to find the GCF is to compare the prime factorization of each number. The prime factorization of 9 is 3 x 3, and the prime factorization of 27 is 3 x 3 x 3. The greatest common factor is the product of the common prime factors, which in this case is 3 x 3 = 9. Another method is to list the factors of each number and find the greatest factor that they have in common. The factors of 9 are 1, 3, and 9, and the factors of 27 are 1, 3, 9, and 27. The greatest factor that they have in common is 9.