Background: Ultra-high dose-rate (UHDR) irradiations (> 50 Gy/s) have recently garnered interest in radiotherapy as they can trigger the so-called “FLASH” effect. To transfer this to clinical radiotherapy treatments, practical tools for online dosimetry need to be developed. Small volume size in ionization chambers may be an interesting characteristic to limit the charge recombination effects. Aims: In that context, we have investigated the charge collection behavior of the recent IBA RazorTM Nano Chamber (RNC) in UHDR pulses to evaluate its potential interest for FLASH radiotherapy. Methods: The RNC Ion Collection Efficiency (ICE) was quantified by performing simultaneous dose measurements under UHDR electron beams with dose-rate independent Gafchromic™ EBT3 films, that were used as the dose reference. A dose-per-pulse (DPP) range from 0.01 to 30 Gy and two pulse duration (1 and 3 µs) were investigated. An empirical logistic model was used to fit the RNC collection efficiency measurements and the results were compared to the Advanced Markus ion chamber. Results: The RNC ICE was found to decrease as the DPP increases, starting from doses above 0.2 Gy/pulse and down to 40% of efficiency at 30 Gy/pulse (Figure). The RNC resulted in a higher ICE for a given DPP in comparison to the Markus chamber, with a measured efficiency found higher than 55% for 10 Gy/pulse versus 25% for the Markus. However, the RNC shows a higher sensitivity to the pulse duration, with a lower efficiency found at 1 µs than at 3 µs, suggesting that this chamber could be more sensitive to the dose-rate within the pulse [1]. Conclusions: The results confirmed that the small sensitive volume of the RNC ensures higher ICE compared to larger chambers, which makes it attractive for online dosimetry of FLASH radiotherapy. [1] Cavallone et al., Med Phys. 2022;1–12. https://doi.org/10.1002/mp.15675