The frequency-dependent absorption characteristics of conventional diesel fuel have been researched in the spectral range of 0.2–1.5？THz by the terahertz time-domain spectroscopy (THz-TDS). The absorption coefficient increased monotonically with the solidifying point of diesel. A nonlinear regression model was established, and the cold flow properties of fuel were presented quantitatively. The results made the solidifying point prediction possible by THz-TDS technology and indicated the bright future in practical application. 1. Introduction Diesel fuel is one of the commercial and industrial fuels, produced by refining crude oil. Because of being heavier and having more carbon content, diesels have some problems when being used in an engine. One of the important problems is a high freezing point that causes block age of filters, and hence, there are some difficulties when they are used in cold conditions. Solidifying point (SP), by which diesel has been classified as different grades of 0#, ？10#, and ？20#, is the highest temperature of diesel loses fluidity in engine, determined on the BSN(S)-4 solid point instruments according to GB/T510-83 [1]. SP is one of the key indicators ensuring the performance of diesel at low temperature, and it should be measured and controlled precisely in practical applications. However, the traditional method for SP has recognized that the test suffered from many disadvantages, some of which include a relatively large fuel sample volume requirement, significant time consumption, and a relatively high reproducibility error. For this reason, there have been many attempts to develop methods to estimate the indexes economically, rapidly, and effectively [2–4]. Serious applications in organism analysis have been proposed in the literature demonstrating that terahertz time-domain spectroscopy (THz-TDS) is an achievable method to detect the diesels via chemical analysis [5–7]. In this work, THz-TDS has been used to investigate the cold flow properties of diesel due to the reason that most terahertz spectrum contains rich physical, chemical, and structural information of the organism, and low-frequency vibrational and rotational spectra of organism are responsible for the cold properties and lie in terahertz frequency range. The results demonstrated the possibility of SP prediction of diesel by THz-TDS. 2. Experimental A repetition rate of 80？MHz, diode-pump mode-locked Ti:sapphire laser (MaiTai, Spectra Physics) provided the femtosecond pulses with duration of 100 fs and center wavelength of 810？nm [8, 9]. A p-type InAs wafer
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