Nano-iron oral supplements emerged as efficient supplements with reduced gastrointestinal side effects. Very recently, nanorods of β-FeOOH was introduced as the most efficient shape of nano-iron to be employed as oral supplement. Developed technologies in the fabrication of nanostructures provides the ability to synthesize β-FeOOH nanorods in various lengths while the other features are constant. As we all know, particles' length has an immense impact on the biologic properties of nanorods. But there are no in vivo data about the impacts of particles length on the bioavailability and possible toxicity of β-FeOOH nanorods. So, in this study, different lengths of β-FeOOH nanorods were fabricated and employed as oral iron supplements. In this order, β-FeOOH nanorods with two lengths (mean length 50 nm and 100 nm) were successfully synthesized via hydrolysis reaction. Oral supplementation of Sprague-Dawley rats with the synthesized nanorods and FeSO