Associated with millimeter-wave (mmWave) 5G, high-gain directional beamforming is being considered as a key technology for overcoming considerable propagation loss. When there is a single dominant propagation path between a transmit- ter and a receiver, this high- gain directional beamforming will be effective. However, when a receiver receives signals from various directions, as is typical in (multipath-rich) mobile environments, increasing directivity gain (by narrowing the beamwidth) at specific directions may block multipath signals from other directions due to the narrowed beamwidth. This will decrease the overall signal power reception. In this paper, we investigate beamwidth-dependent directional propagation loss behaviors based on 28 and 38 GHz measurements. The measurement data were collected in a typical urban micro-cellular environment where many objects and surrounding buildings compose multipath-rich propagation environments. To save measurement time and efforts, we conducted the measurements with a narrow 10^o HPBW (half power beamwidth) antenna and then synthesized other beamwidth characteristics. Our results show that there is an inversely proportional relationship between the beamwidth and the power loss compared to the omnidirectional antenna reception. Although we analyzed with limited data only at 28 and 38 GHz, we believe that these characteristics will be helpful in developing mmWave 5G systems.