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Napier grass (Pennisetum purpureum Schumach) has been identified as a potential energy crop in Thailand. However, information regarding the biomass production characteristics of napier grass in response to K and N fertilization is required to guide management decisions for biofuel feedstock. Our objective was to determine the effects of K and N rates on biomass yield, developmental morphology, nutrient content and removal. This experiment was conducted during 2016 and 2017 at Kamphaeng Saen (KPS), Thailand. The experimental design was a split plot arranged in a randomized complete block (RCB) with four replications. Three K fertilization rates (0, 250, and 500 kg ha−1) were used for the main plots and four N fertilization rates (0, 250, 500, and 1000 kg ha−1) were arranged as subplots. With increasing N fertilization rate, most growth parameters increased, except leaf to stem ratio and dry matter concentration decreased. Total dry matter yield (TDMY) increased from 16.6 to 43.0 t ha−1 and from 15.2 to 41.6 t ha−1 in 2016 and 2017, respectively, as N rate increased from 0 to 1000 kg ha−1. However, growth and TDMY were not affected by K fertilizer. Increased TDMY resulted from an increase in the proportion of elongating tillers leading to advancing mean stage by count (MSC) from 2.35 to 2.45. Therefore, although napier grass demonstrated large biomass production under multiple harvest regimes, its higher removal rates (285.0–1615. kg K ha−1 and 86.7–422.2 kg N ha−1) were varied with fertilization rates indicating a potential for higher fertilizer requirements and production costs over time. [ABSTRACT FROM AUTHOR]