We have investigated experimentally and theoretically the imaging performance of our newly constructed in-line holography x-ray phase-contrast imaging system with an ultrafast laser-based x-ray source. Projection images of nylon fibers with diameters in the 10-330 μm range were obtained using an ultrafast (100 Hz, 28 fs, 40 mJ) laser-based x-ray source with Mo and Ta targets and Be filter, and Gaussian spatial-intensity distribution (FWHMS = 5 μm). A cooled CCD camera (24 μm pitch) with a Gd 2OS 2 screen coupled via 1:1 optical taper was used (FWHMD = 50 μm). We have investigated nylon-fiber image quality vs. imaging setup geometry and x-ray spectra. The following parameters were evaluated: contrast, signal-to-noise ratio (SNR), resolution, and sampling. In addition, we performed theoretical simulation of image formation for the same objects but within a wide range of geometrical parameters. The rigorous wave-optical formalism was used for modeling of the free-space propagation of x-rays from the object plane to the detector, and the "projection approximation" was used. We found reasonable agreement between predictions of our analytical model and the experiments. We conclude that: a) Optimum magnification maximizing contrast and SNR is almost independent of the source-to-detector (R) distance and depends strongly on the diameter of the fiber, b) The corresponding maximum values of the contrast and SNR are almost linear with respect to R; the optimum magnification decreases with fiber diameter, c) The minimum diameter of fiber defines the minimum source-to-object distance R 1 if R is fixed and the object is moved.