Diffuse Ionized Gas (DIG) is prevalent in star-forming galaxies. Using a sample of 365 nearly face-on star-forming galaxies observed by MaNGA, we demonstrate how DIG in star-forming galaxies impacts the measurements of emission line ratios, hence the interpretation of diagnostic diagrams and gas-phase metallicity measurements. At fixed metallicity, low ΣHα regions display enhanced [S ii]/Hα, [N ii]/Hα, [O ii]/Hβ, and [Oi]/Hα. [Sii]/Hα is determined by metallicity gradient and ΣHα. In line ratio diagnostic diagrams, contamination by DIG moves H ii regions towards composite or LI(N)ER-like regions. A harder spectrum is needed to explain DIG line ratios. Leaky H ii region fails to explain the composite/LI(N)ER line ratios. Our result favors ionization by evolved stars as a major ionization source for DIG with LI(N)ER-like emission. DIG can significantly bias the measurement of gas metallicity and metallicity gradient derived using strong line methods. Metallicities derived using N2O2 is optimal because it exhibits the smallest bias and error. Using O3N2, R23, N2=[Nii]/Hα, and N2S2Hα (Dopita et al. 2015) to derive metallicities will introduce a bias the metallicity gradient as large as the metallicity gradient itself. IZI (Blanc et al. 2015) can not be applied to DIG to get accurate metallicity because it currently contains only H ii region models fail to describe DIG.