This study indicates the importance of structure optimization and analysis for investigating direct screenprinted pastes of carbon based microstructures for use in flexible thermal interface material (TIM). The basalt fiber that is used as reinforcement in microstructures is a non-toxic as a mineral fiber made from natural raw material. After direct screen printing, the effect of the basalt particles on the morphology and surface of the screen-printed pastes was analyzed by SEM observation. Basalt particles support densified structures of which the contact surface gaps were reduced to optimize the conductive interface materials. xGnP paste reinforced with well-dispersed basalt particles was used to optimize the surface onto substrate by decreasing the void volume. This result could contribute to improvement of the thermal properties owing to decreased thermal resistance at the interface by filling gaps. This is because, thermal conduction is sensitive to the presence of surface structures such as voids and defects. The in-plane thermal conductivity of xGnP paste containing a high concentration of basalt particles was increased to 3.011 W m-1K-1, representing an improvement of 108 % compared to a pure xGnP paste. The high concentrations of basalt particles densified the microstructures, and served as reinforcement to enhance the thermal performance of the thin TIM. This approach will be extended to create sustainable pastes for effective thermal management and for optimization of a direct printing process for thermal conductive interface materials.