Monoclinic molybdenum trioxide (β-MoO 3 ) nanostructures (shaped like nanoribbons: NRs) were grown on Si(1 0 0), Si(5 5 1 2) and fluorine-doped tin oxide (FTO) by molecular beam epitaxy (MBE) technique under ultra-high vacuum (UHV) conditions. The dependence of substrate conditions and the effective thickness of MoO 3 films on the morphology of nanostructures and their structural aspects were reported. The electron microscopy measurements show that the length and the aspect ratio of nanostructures increased by, 260% without any significant change in the width for a change in effective thickness from 5 nm to 30 nm. NRs are grown along 〈0 1 1〉 for all the effective thickness of MoO 3 films. Similarly, when we increased the film thickness from 5 nm to 30 nm, the optical band gap decreased from 3.38 ± 0.01 eV to 3.17 ± 0.01 eV and the local work function increased from 5.397 ± 0.025 eV to 5.757 ± 0.030 eV. Field emission turn-on field decreased from 3.58 V/μm for 10 μA/cm 2 to 2.5 V/μm and field enhancement factor increased from 1.1 × 10 4 to 5.9 × 10 4 for effective thickness variation of 5–30 nm β-MoO 3 structures. The β-MoO 3 nanostructures found to be much better than the α-MoO 3 nanostructures due to low work function, low turn on field and high field enhancement factor, and are expected to be useful applications.