Compression ignition of hydrogen engines with a homogeneous pre-mixture is a promising method to enhance the thermal efficiency as well as to reduce unique NOx exhausted from the engine due to spatial reaction of the mixture. However, hydrogen gas has a relatively high self-ignition temperature. Therefore, compression ignition for a neat hydrogen-air pre-mixture is considered impossible to achieve without additives. Research on this has not yet been attempted for this reason. In order to initiate the development of a compression ignition engine operated under a neat hydrogen-air pre-mixture and room temperature conditions, this paper evaluates the feasibility of compression ignition by using a high compression ratio and analyzes the characteristics such as abnormal combustion and operation region. It is found that the neat hydrogen-air pre-mixture is self-ignited at a high compression ratio without any assisting method in room temperature, thus refuting the preconception that compression ignition of hydrogen engine is impossible. A compression ratio of at least around epsilon = 32 is required to self-ignite the engine under cold start and decreases to epsilon = 26 with an increased equivalence ratio under firing conditions. The minimum equivalence ratio for compression ignition is in the region of phi = 0.11-0.22 of an ultra lean mixture for all operating conditions. The operation region at each compression ratio is extremely narrow (around phi = 0.04-0.06) due to knocking and backfire resulting from the high compression ratio. Copyright (c) 2012, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights reserved.