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In the optically thin regime, the intensity ratio of the two Si IV resonance lines (1394 and 1403 Å ) are theoretically the same as the ratio of their oscillator strengths, which is exactly 2. Here, we study the ratio of the integrated intensity of the Si IV lines ($R=\int I_{1394}(λ)\mathrm{d}λ/\int I_{1403}(λ)\mathrm{d}λ$) and the ratio of intensity at each wavelength point ($r(Δλ)=I_{1394}(Δλ)/I_{1403}(Δλ)$) in two solar flares observed by the Interface Region Imaging Spectrograph. We find that at flare ribbons, the ratio $R$ ranges from 1.8 to 2.3 and would generally decrease when the ribbons sweep across the slit position. Besides, the distribution of $r(Δλ)$ shows a descending trend from the blue wing to the red wing. In loop cases, the Si IV line presents a wide profile with a central reversal. The ratio $R$ deviates little from 2, but the ratio $r(Δλ)$ can vary from 1.3 near the line center to greater than 2 in the line wings. Hence we conclude that in flare conditions, the ratio $r(Δλ)$ varies across the line, due to the variation of the opacity at the line center and line wings. We notice that, although the ratio $r(Δλ)$ could present a value which deviates from 2 as a result of the opacity effect near the line center, the ratio $R$ is still close to 2. Therefore, caution should be taken when using the ratio of the integrated intensity of the Si IV lines to diagnose the opacity effect.