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We present spectroscopy of 28 SNR candidates as well as one H II region inM81, and two SNR candidates in M82. Twenty six out of the M81 candidates turnout to be genuine SNRs, and two in M82 may be shocked condensations in thegalactic outflow or SNRs. The distribution of [N II]/H{\alpha} ratios of M81SNRs is bimodal. M81 SNRs are divided into two groups in the spectral lineratio diagrams: an [O III]-strong group and an [O III]-weak group. The latterhave larger sizes, and may have faster shock velocity. [N II]/H{\alpha} ratiosof the SNRs show a strong correlation with [S II]/H{\alpha} ratios. They show aclear radial gradient in [N II]/H{\alpha} and [S II]/H{\alpha} ratios: dLog ([NII]/H{\alpha})/dLog R = -0.018 {\pm} 0.008 dex/kpc and dLog ([SII]/H{\alpha})/dLog R = -0.016 {\pm} 0.008 dex/kpc where R is a deprojectedgalactocentric distance. We estimate the nitrogen and oxygen abundance of theSNRs from the comparison with shock-ionization models. We obtain a value forthe nitrogen radial gradient, dLog(N/H)/dLogR = -0.023 {\pm} 0.009 dex/kpc, andlittle evidence for the gradient in oxygen. This nitrogen abundance shows a fewtimes flatter gradient than those of the planetary nebulae and H II regions. Wefind that five SNRs are matched with X-ray sources. Their X-ray hardness colorsare consistent with thermal SNRs.

Optical Spectroscopy of Supernova Remnants in M81 and M82