A primary radiator for 2-dimensional parabolic reflectors was investigated for millimeter-wave planar antenna, which was constructed by a slotted array fed by an oversized waveguide with the 2-dimensional parabolic reflector. The primary radiator consists of the vertical strip transmission line on which an L-shaped array pattern was etched. At first, the length of an L-shaped pattern was decided by calculating the reflection characteristics. Next, spacing between two L-shaped patterns was chosen by calculating radiation patterns of the L-shaped array. It was obvious that a broad radiation pattern was obtained by selecting the spacing. In addition, a stub was installed on the vertical strip transmission line to suppress the reflection from this radiator. Based on the calculated and measured radiation patterns of the radiator, it was confirmed that this radiator would be suitable for applications to a primary radiator for MMIC at millimeter wavelengths.