This chapter focuses on understanding lamina through micromechanics and obtaining key mechanical properties using the rule of mixtures. It investigates lamina behavior by integrating micromechanics principles. Lamina, a fundamental building block in composite materials, exhibits intricate mechanical properties that significantly influence the overall performance of composites. This chapter employs micromechanics to explore the internal mechanisms governing lamina behavior at the microscopic level. By examining the interactions between individual constituents, the study aims to explain the intrinsic factors influencing lamina properties. Furthermore, the rule of mixtures, a widely used method to estimate global mechanical properties of composites based on the characteristics of constituent components, is presented. The application of the rule of mixtures enables a macroscopic understanding of lamina behavior by considering the contributions of individual components, both matrix and reinforcement. The influence of the amount of reinforcement on mechanical properties (i.e., modulus of elasticity, tensile strength, etc.) for both the direction along the applied load and transverse to the load is also discussed in this chapter. Moreover, from mechanical properties estimated for the longitudinal direction, properties for other lamina with different orientations can be estimated using the transformation matrix approach. The disadvantages and limitations of the rule of mixtures are presented, and to verify the accuracy of results, several tests are recommended to implement on the mechanical properties. More research on the rule of mixtures is needed to obtain a better fit with test results on mechanical properties. Finally, atomistic simulations are important numerical methods for the estimation of mechanical properties of material, but there are limitations in capturing the exact behavior of atoms under certain load excitation through computer simulation, which will make nanomechanics analysis difficult to implement.