Field experiments were carried out in split plot design during the dry and wet seasons for two years (two seasons each in 2016–2017 and 2017–2018) with two genotypes (SH4 and SUIN053), two plant geometry (30×15 cm and 45×15 cm main plots) and three levels of NPK (20 kg N ha^–1, 40 kg P ha^–1 and 40 kg K ha^–1; 20 kg N ha^–1, 60 kg P ha^–1 and 60 kg K ha^–1; 20 kg N ha^–1, 80 kg P ha^–1 and 80 kg K ha^–1) with an objective to study the relationship between fibre yield of sunhmep and thermal indices.  The results indicated that the thermal units such as cumulative heat unit (CHU), photo thermal unit (PTU) and  helio thermal unit (HTU) were the highest during dry seasons, while relative temperature disparity (RTD) was the highest during wet seasons irrespective of the genotypes, plant geometry and fertilizer levels.  The combined analysis of variance showed that the suitability of sunnhemp genotypes for obtaining fibre and seed yields varied with season.  The results further indicated that sunnhemp grew during dry seasons with longer photoperiod and higher values of growing degree days (GDD), HTU and PTU resulted in a higher fibre yield, while a higher seed yield and relatively longer, finer and stronger fibres were obtained during wet seasons with higher RTD values.  Regression analysis indicated that CHU was positively related to fibre yield, while RTD was positively related to seed yield.   CHU beyond 2 000 °C d reduced seed yield and favoured fibre production.  In contrary to CHU, RTD values were positively related to seed yield and negatively related to fibre yield.   Similarly, HTU had an inverse relationship with fibre yield while PTU had a positive relationship with fibre yield.  The genotype SH4 produced a seed yield of 1 361 kg ha^–1 during wet seasons, which was significantly higher than SUIN053, while a fibre yield of 990 kg ha^–1 (significantly higher than that of SH4) was obtained for SUIN053 that required less CHU to attain the phenological events during dry seasons.  The per unit area yields of seed and fibre with the closer spacing (30 cm×15 cm)  by virtue of higher plant density were 17.0 and 14.9% higher than those with the spacing of 45 cm×15 cm, respectively.  Higher doses of P and K resulted in higher seed and fibre yields.