The influences of temperature change on the states of n-alkanes (n-undecane(C11), n-dodecane(C12), n-tridecane(C13) and n-tetradecane(14)) were studied by using terahertz time-domain spectroscopy. The terahertz time-domain spectra were obtained by scanning the samples at different temperatures. The spectra of frequency-domain, refractive index and absorption coefficient obtained after data processing could reflect the state change of n-alkanes. When the temperature dropped below its freezing point, the states of n-alkanes were converted from liquid to solid crystals causing the effective frequency range in frequency-domain spectra to narrow and the area of frequency-domain to decrease. The decrease of temperature could result in the crystallization of alkane molecules, the increase of refractive index and the linear dependence of absorption coefficient on frequency. The experimental results showed that terahertz time-domain spectroscopy could be used to detect the non-polar hydrocarbons represented by normal alkanes.