In this study, PM2.5 mass readings by a portable light scattering monitor (Dust Trak) were compared to those obtained using a collocated beta gauge monitor (E-BAM) at various ambient relative humidities. The mass readings of two automatic particle monitors were also compared with those of a manual sampler. The inlet heater of the beta gauge was actuated or switched off to determine whether ambient relative humidity has an effect on the beta gauge readings. This work demonstrated that PM2.5 readings of the light scattering monitor were higher than those of the beta gauge monitor, regardless of whether the inlet heater was turned on or off. In addition, the mean PM2.5 readings of the light scattering monitor were also higher than those of the manual sampler. Results indicated that the relative humidity affected PM2.5 reading of the light scattering monitor and PM2.5 reading increased as the relative humidity rose. In contrast, when the inlet heater of the beta gauge was switched on, the mean PM2.5 mass reading of beta gauge monitor was close to that of the manual sampler, and it was lower than that of the light scattering monitor. The inlet heater appeared to avoid the relative humidity effect on PM2.5 mass readings of the beta gauge monitor.