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In the optically thin regime, the intensity ratio of the two Si IV resonancelines (1394 and 1403 \AA\ ) are theoretically the same as the ratio of theiroscillator strengths, which is exactly 2. Here, we study the ratio of theintegrated intensity of the Si IV lines ($R=\intI_{1394}(\lambda)\mathrm{d}\lambda/\int I_{1403}(\lambda)\mathrm{d}\lambda$)and the ratio of intensity at each wavelength point($r(\Delta\lambda)=I_{1394}(\Delta\lambda)/I_{1403}(\Delta\lambda)$) in twosolar flares observed by the Interface Region Imaging Spectrograph. We findthat at flare ribbons, the ratio $R$ ranges from 1.8 to 2.3 and would generallydecrease when the ribbons sweep across the slit position. Besides, thedistribution of $r(\Delta\lambda)$ shows a descending trend from the blue wingto the red wing. In loop cases, the Si IV line presents a wide profile with acentral reversal. The ratio $R$ deviates little from 2, but the ratio$r(\Delta\lambda)$ can vary from 1.3 near the line center to greater than 2 inthe line wings. Hence we conclude that in flare conditions, the ratio$r(\Delta\lambda)$ varies across the line, due to the variation of the opacityat the line center and line wings. We notice that, although the ratio$r(\Delta\lambda)$ could present a value which deviates from 2 as a result ofthe 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 integratedintensity of the Si IV lines to diagnose the opacity effect.

Diagnosing the Optically Thick/Thin Features Using the Intensity Ratio of Si IV Resonance Lines in Solar Flares