  We propose Fast Radio Burst (FRB) timing, which uses the precision
measurements of the arrival time differences of repeated FRB signals along
multiple sightlines, as a new probe of gravitational waves (GWs) around nHz to
$\mu$Hz frequencies, with the highest frequency limited by FRB repeating
period. The anticipated experiment requires a sightline separation of tens of
AU, achieved by sending radio telescopes to space. We find the signal of
arrival time difference induced by GWs depends only on the local GWs in the
solar system and we can correlate the measurements from different FRB sources
or the same source with different repeaters, which leads to a better
sensitivity with a larger number of FRB repeaters detected. The projected
sensitivity shows this method is a competitive probe in the nHz to $\mu$Hz
frequency range. It can fill the '$\mu$Hz gap' between pulsar timing arrays and
Laser Interferometer Space Antenna (LISA) and is complementary to other
proposals of GW detection in this frequency band.
