The polynomial bounds of Jordan's inequality, especially the cubic and quartic polynomial bounds, have been studied and improved in a lot of the literature; however, the linear and quadratic polynomial bounds can not be improved very much. In this paper, new refinements and improvements of Jordan's inequality are given. We present new lower bounds and upper bounds for strengthened Jordan's inequality using polynomials of degrees 1 and 2. Our bounds are tighter than the previous results of

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... mials of degrees 1 and 2. More importantly, we give new improvements of Jordan's inequality using polynomials of degree 5, which can achieve much tighter bounds than those previous methods.