Big bang nucleosynthesis in a modified gravity model of $f(R)propto R^n$ is investigated. The only free parameter of the model is a power-law index $n$. We find cosmological solutions in a parameter region of $1< n leq (4+sqrt{6})/5$. We calculate abundances of $^4$He, D, $^3$He, $^7$Li, and $^6$Li during big bang nucleosynthesis. We compare the results with the latest observational data. It is then found that the power-law index is constrained to be $(n-1)=(-0.86pm 1.19)times 10^{-4}$ (95 % C.L.) mainly from observations of deuterium abundance as well as $^4$He abundance.
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