Working media (typically, molten salt and synthetic oil) play a blood role in transporting solar energy for the concentrating solar power (CSP) technology. Yet, transient characteristics of both molten salt and synthetic oil are far less understood. Herein, a one-dimensional transient model associating with SIMPLE algorithm is developed to comparatively investigate the response characteristics of the parabolic-trough collector with molten salt (Solar Salt) and synthetic oil (Therminol VP-1). It has been demonstrated that although molten salt that takes advantages of high heat capacity, high thermal stability and high operation temperature is capable of building the higher-efficiency CSP plant, an intriguing finding shows that molten salt possesses much longer response delay than synthetic oil does. It is found that as DNI increases from 400 W m(-2) to 1000 W m(-2), the outlet temperature of loop with molten salt exhibits up to 3.4 times of response delay (from 811 s to 3285 s) than that with synthetic oil (from 300 s to 963 s). Likewise, when mass flowrate varies, the outlet temperature of loop with molten salt shows up to 4 times of response delay (from 546 s to 3373 s) than that with synthetic oil (from 187 s to 836 s). It is also found that the response delay is exponentially correlated to the variations of DNI and mass flowrate as y = A.exp(-x/tau) + B for both molten salt and synthetic oil loops. These findings remind of a remarkable drawback of molten salt in the views of responsivity and controllability, which has been seldom pointed out before.