The signal visibility characteristics of spatial filtering velocimetry using a transmission grating have been investigated theoretically and experimentally. It is found that the visibility of output signals is given as a function of the ratio of both the size and the separation of particles to the grating period in a certain restricted case. In general, it also depends on both the line number of the grating and its geometrical shape. In order to make the signal visibility always usable for the sizing of individual particles, the effect of the spatial particle density on the visibility must be minimized.