The notion of an effective longitudinal coherence length with its value much greater than λ2/(2Δλ) has been adopted in small-angle X-ray scattering communities for years, where λ and Δλ denote the incident wavelength and its spread, respectively. Often the implications of the effective longitudinal coherence length do not even enter considerations in the designing and data treatment of small-angle scattering experiments. In this work, conventional transmission small-angle X-ray scattering (tSAXS) was performed to reveal a clear angular dependence on effective longitudinal coherence length. The measured values of effective longitudinal coherence length can be as high as one millimeter, whereas the value of calculated λ2/(2Δλ) is in nanometers.