We present numerical simulations in order to investigate the resonances in reflection, transmission, and absorption of surface plasmons for triangular gratings. The results reveal a number of resonances depending of grating geometrical parameters, surface materials, and characteristics of incident light. For metals Cu, Ag, Au, Ni, Pd, and Pt, the intensive weakly flat grating-resonance at TM-polarized incident light under normal direction is at wavelength $$\lambda = 0.83 \pm 0.01\,\upmu{\text{m}}$$ λ = 0.83 ± 0.01 μ m , and a weaker grating resonance is at wavelength around $$0.46\,\upmu{\text{m}}$$ 0.46 μ m . For strongly flat grating weak resonances appear for some metals, and disappear for others.