New cancer treatment modalities are being developed in order to further improve the dose delivery to the tumor while sparing healthy tissues. Such a gain could be obtained by further optimizing the spatial dose distribution for example using mini-beams. Another technique based on temporal optimization, called FLASH therapy, which uses ultra high dose rates, is currently under development. For these particular beams instantaneous dose-rates or doses-per-pulse are up to several orders of magnitude greater than those produced by conventional radiation sources. Conventional active dosimeters (diode or ionization chamber) saturate in the ultra high dose rates (UHDR) irradiation, consequently on-line dosimetry of the beam cannot easily be performed with such devices. CEA-LIST, in the frame of ASTROLABE project (funded by INSERM) and in collaboration with Institut Curie, is developing a dedicated dosimeter for FLASH beams, based on optimised fast plastic scintillator coupled to a silicon photomultiplier (SiPM) sensor.   A set-up made of a homemade formulated plastic scintillator, tuned in response time and wavelength emission, a wave guide, a SiPM and its electronic readout is produced. This device is tested on standard therapy beams at DOSEO plateform in CEA-Saclay and with electrons FLASH beams at Institut Curie in Orsay. The Cerenkov contribution is corrected for all measurements by using twin set-up without plastic scintillator. In this work, we will present for the first time experimental results obtained with this optimised fast plastic scintillator coupled to SiPM sensor in FLASH beams : modification of the choice of scintillator used and the assembling phase is presented with the characterisation in laboratory using radioactives sources and standard therapy beams. Following this step, first characterisation results obtained with FLASH electron beams are presented and compared to standard devices.