Purpose: To overcome side effects of repetitive administration of Diazepam (Dzp) besides gaining benefits from sustaining release (SR) of the drug, which contributes to patient compliance, we concentrated on designing and preparing Dzp Solid Lipid Nanoparticles (SLNs). Methods: Using cholesterol (CHOL), stearic acid (SA) and glycerol monostearate (GMS), SLNs were prepared by high shear homogenization technique coupled with sonication. Polysorbate 80 (Tween 80) was used as a nonionic surfactant. After modification of prepared SLNs, particle size, zeta potential, drug-loading efficiency, morphology and scanning calorimetry as well as release studies were conducted. To increase the stability of desired particles, freeze-drying by cryoprotectant was carried out. In the final stage, In-vivo study was performed by oral (PO) and intraperitoneal (IP) administration to Wistar male rats. Results: Results indicated that optimized prepared particles were in average 150 nm diameter in spherical shape with 79.06 % loading efficiency and release of more than 85% of loaded drug in 24 hours. In-vivo investigations also illustrated differences in blood distribution of Dzp after loading this drug into SLNs. Conclusion: Based on the findings, it seems that drug delivery using SLNs could be an opportunity for solving complications of Dzp therapy in future.