Sure, just calculate the total number of molecules or formula units per unit cell, and from all the lattice parameters (including angles), calculate the volume per unit cell. Divide unit cell volume by number of formula units per unit cell to get volume per formula unit. Do that for each crystal structure. Then divide volume per formula unit of one structure by that of the other structure; that ratio is the volumetric mismatch between the two structures.

Dear Hemendra Chouhan please be a bit more specific in your question.

I assume you are interested in epitaxial growth of a monoclinic layer on top of a hexagonal. Thus the lattice mismatch would refer to the relative length of hexagonal [uv0] vectors in comparison to monoclinic vectors. Do you know anything about the relative orientation of the two lattices?

In any lattice the length of a vector is calculated from

d^2 = v^T*G*v

where v^t = [u, v, w] is a row matrix, the transposed of the components of the lattice vector v, which is a 3x1 column matrix. G is the metric tensor, a symmetric 3x3 matrix whose elements are the scalar products of the base vectors.

thus G(1,1) = a^2; G(1,2) = a*b*cos(gamma); g(1,3) = a*c*cos(beta); G(2,3) = b*c*cos(alpha); etc

Angles between vectors are best calculated using the definition of the scalar product:

vector(u)*vector(v) = |u|*|v| * cos(angle between u and v)

The lengths of the vectors is available from d^2 = v^T*G*v

R. B. Neder Good job making your assumptions explicit.

I should have mentioned that I'm thinking about a precipitate, so you have crystallites of one phase embedded in the other.