The lamellar peritectic coupled growth in Fe-Ni peritectic system was investigated using the equilibrium Boettinger-Jackson-Hunt model. It was found that the slope of the undercooling vs. lamellar spacing is very near zero around the minimum overheating, and the coupled growth can exist under this condition even if the slope of the undercooling vs. lamellar spacing curve is slightly smaller than zero. In addition, the peritectic reaction can never reach completion during the peritectic coupled growth. So the equilibrium peritectic coupled growth was modified by considering the incompletion of the peritectic reaction. It was shown that when the fractions of the peritectic reaction reach 60%-80% completion, the calculated undercooling vs. lamellar spacing curves agree well with the experimental observations in the directionally solidified Fe-Ni alloys.