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The notion of an effective longitudinal coherence length with its value much greater than <inline-formula><math display="inline"><semantics><mrow><msup><mi>λ</mi><mn>2</mn></msup><mo>/</mo><mrow><mo>(</mo><mrow><mn>2</mn><mi mathvariant="normal">Δ</mi><mi>λ</mi></mrow><mo>)</mo></mrow></mrow></semantics></math></inline-formula> has been adopted in small-angle X-ray scattering communities for years, where <inline-formula><math display="inline"><semantics><mi>λ</mi></semantics></math></inline-formula> and <inline-formula><math display="inline"><semantics><mrow><mi mathvariant="normal">Δ</mi><mi>λ</mi></mrow></semantics></math></inline-formula> 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 <inline-formula><math display="inline"><semantics><mrow><msup><mi>λ</mi><mn>2</mn></msup><mo>/</mo><mrow><mo>(</mo><mrow><mn>2</mn><mi mathvariant="normal">Δ</mi><mi>λ</mi></mrow><mo>)</mo></mrow></mrow></semantics></math></inline-formula> is in nanometers.