Operation of a chopper‐prebuncher high voltage injection system and a high gradient 17 GHz linac having an inherent ability to generate 1 degree (162 fs) electron bunches emphasized the need for a simple means of accurately measuring very short electron bunches while also providing on‐line guidance for optimizing the injector and linac bunch length controls. To meet this need, a fast diagnostic for the direct measurement of short bunch longitudinal phase space, with a time resolution of less than 100 fs, has recently been installed on the 17 GHz linac at the MIT Plasma Science and Fusion Center. The diagnostic system, having a high field (0.2 MA/m) circularly polarized 17 GHz beam deflector, is described; images of 17 GHz electron bunches are shown; and bunch charge distributions mapped with a matrix of 150 micron diameter collimators spaced on 240 micron centers (sampling intervals of 40 fs and 0.4&percent; &Dgr;p/p) are presented. The minimum time resolution of this fast diagnostic device is linearly dependent on the emittance limited beam spot size and is, therefore, especially suited for very low beam emittance, ultra short bunch applications. Direct and simple calibration, real time viewing and extreme phase coherence of the RF system are added advantages of this time domain diagnostic. © 2004 American Institute of Physics