The microstructure and mechanical properties of Mg-4.5Al-2.5Zn-0.3Mn-0.2Ca (wt.%, designated as AZ42) alloys in extruded (at extrusion ratios of 28, 20 and 11.5) and peak-aged states were investigated, by using optical microscopy, scanning electron microscopy, energy dispersive spectrometry and electron backscatter diffraction. The results show that extrusion produces a typical basal fiber texture and streamlines of second phases. All samples exhibit the lowest Schmid factor of basal slip (SF_b) and the superior tensile yield strength (TYS) along extrusion direction (ED). The sample with extrusion ratio of 20 exhibits the largest average grain size, but the smallest SF_b which compensates for the disadvantage of grain coarsening and maintains the strength. After being peak-aged at 175 °C for 48 h, the sample with the extrusion ratio of 20 shows the optimal TYS along all the directions, compared to the other samples. This hopes to provide useful information for optimizing the deformation parameters of the AZ42 alloys.