Monoclinic molybdenum trioxide (beta-MoO3) nanostructures (shaped like nanoribbons: NRs) were grown on Si (1 0 0), Si(5 5 1 2) and fluorine-doped fin oxide (FTO) by molecular beam epitaxy (MBE) technique under ultra-high vacuum (UHV) conditions. The dependence of substrate conditions and the effective thickness of MoO3 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 MoO3 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/mu m for 10 mu A/cm(2) to 2.5 V/mu m and field enhancement factor increased from 1.1 x 10(4) to 5.9 x 10(4) for effective thickness variation of 5-30 nm beta-MoO3 structures. The beta-MoO3 nanostructures found to be much better than the alpha-MoO3 nanostructures due to low work function, low turn on field and high field enhancement factor, and are expected to be useful applications.