A novel method to measure in-plane resolution (modulation transfer function, or MTF) and slice thickness (slice sensitivity profile, or SSP) of a digital radiographic tomosynthesis system is presented. With this method, one can measure these two important system IQ characteristics simultaneously without suffering from incontinuous sampling, aliasing, and partial volume effect as do the existing methods. The method is based on imaging a shallow-angled slice ramp phantom. The MTF is measured as the HWHM of the Fourier transformation of the first derivative of edge profiles. The HWHM corresponding to the sharpest of edge profile represents the in-plane resolution of the system, and the slice thickness of the system is determined from the HWHM vs. z-distance curve. The in-plane resolution result has been confirmed by the measurement from an animal skull specimen. The experiment results have shown that, for a typical 40-degree sweep, 61 projections, and using a Specialized Filtered Backprojection (SFBP) algorithm, the in-plane resolution of the measured system is close to 1 lp/mm (as measured by the HWHM of MTF), and effective slice thickness is 1.7 mm and 4.0 mm at HWHM and HW3TM, respectively. It is also observed that, while the in-plane resolution remains constant between planes at 7 cm and 30 cm above the detector plane, SSP has increased (i.e., slice thickness increased) 20% on average with the increase of the plane height. We demonstrate one of the applications of the method to optimize the sweep angle of a tomosynthesis system. The results show that, in a typical angular range from 20 to 60 degrees, the increase in sweep angle can intrinsically reduce slice thickness but less significantly impact in-plane resolution.