Abstract: Modern aerospace gas turbines require large numbers of small diameter holes ( < 1 mm ) to provide cooling in the turbine blades, nozzle guide vanes, combustion chambers and afterburner. A typical modern engine will have ~ 100,000 such holes. Drilling these cooling holes by Nd-YAG laser is now well established. Such holes can be successfully produced by laser trepanning, but this is a relatively slow process compared with laser percussion drilling both drilling techniques are performed using direct free space mirror delivery system. The aerospace industry would like to move to a fibre optic delivery system, but the only fibres capable of carrying the high peak powers involved are 600 µm core fibres with the associated poor beam and poor hole quality. By reducing the peak power of the laser and careful control of laser pulse and drilling parameters useful hole with good hole quality can be achieved. This paper investigates these parameters using a GSI Lumonics new JK700 series laser with a 400 µm optical fibre delivery system.