Diffusion Ordered SpectroscopY (DOSY) is a powerful technique to measure translational diffusion coefficients of various samples, such as small organic compounds, proteins, nucleic acids, and mixtures of them. The DOSY experiment, however, is very sensitive to thermal convection caused by low air flow in a probe head, especially in a cryogenic-probe head, producing unwanted fake broad peaks in the DOSY spectra. The fake peaks render it difficult to analyze the DOSY spectra especially for mixture samples because the spectral separation becomes worse. Some methods to suppress the thermal convection are proposed, but they cannot suppress the thermal convection sufficiently, or require irregular NMR tubes. Therefore, a simple method to effectively suppress the thermal convection is desired. In this study, we demonstrate that insertion of eighteen 0.8-mm diameter capillaries in a 5-mm diameter regular NMR tube effectively prevents the thermal convection. The inserted capillaries, which are parallel to the NMR sample tube axis, make small compartments in the NMR tube, and prevent the thermal convection. Hence, the sample solution would move according to natural diffusion phenomena. The mixture of Quinine and m-Cresol in deuterated chloroform was used to monitor the thermal convection. At a low air flow rate (270 L/h), fake broad peaks were observed without capillaries, whereas two clearly separated peaks, corresponding to Quinine and m-Cresol, were observed with capillaries even in the same condition. It should be noted that the capillaries did not perturb the Z-axis magnetic field homogeneity. Since this method is very simple and requires no special tool, it can be widely applied to NMR samples that are influenced by the thermal convection.