In this study, we calibrated the performance of low-cost and portable PM sensors under the condition of steady-state particle mass concentration by challenging them with lab-generated particles in different size distributions and compositions. Tested PM sensors included four low-cost optical sensors, i.e., Sharp, Shinyei, Samyoung and Oneair, TSI DustTrak and Personal Dust Monitor (PDM). A scientific Tapered Element Oscillating Microbalance (TEOM; Thermal Scientific Model 1405) was applied as the reference. Same as what concluded in previous studies, our calibration data show that the readouts of all tested PM sensors had linear relationships with the particle mass concentration when the size distribution and composition of test particles were kept unchanged. However, the linear calibration lines for four low-cost PM sensors and DustTrak are different when the size distribution (i.e., mean size and geometrical standard deviation) and composition of particles were varied. The PDM output in general agreed well with that of reference TEOM for particles with mass concentration higher than 300 mu g/m(3). The PDM, however, lost its sensitivity for measuring particles in the mass concentration less than 300 mu g/m(3). Our study also demonstrated that the calibration of low-cost PM sensors should be performed under the condition of steady particle mass concentration instead of under the condition of transient particle concentration, even at very slow transient situation.