Let q be a prime number and K = Q(θ) be an algebraic number field with θ a root of an irreducible trinomial x 6 +ax+b having integer coefficients. In this paper, we provide some explicit conditions on a, b for which K is not monogenic. As an application, in a special case when a = 0, K is not monogenic if b ≡ 7 mod 8 or b ≡ 8 mod 9. As an example, we also give a non-monogenic class of number fields defined by irreducible sextic trinomials.Throughout the paper, Z K denotes the ring of algebraic integers of an algebraic number field K. For a prime number q and a non-zero m belonging to the ring Z q of q-adic integers, v q (m) will be defined to be the highest power of q dividing m.