The experiment was conducted to evaluate the effect of harvesting day and plant spacing on agronomic performance and chemical composition of Brachiaria hybrid cv. Mulato II grass under irrigation condition at Kudmi site, North Mecha District, Ethiopia. The experiment was a 3x4 factorial layout in RCBD with three replications each contains a total of 12 plots resulted a total of 36 plots and the net harvestable plot area was 9m² (3m*3m). The harvesting day were at 75, 90 and 105 day and plant spacings used were 20cm, 30cm, 40cm and 50cm between plants. Morphological characteristics such as plant height (PH), number of tillers per plant (NTPP), numbers of leaf per plant (NLPP), leaf length per plant (LLPP), number of internodes per tiller (NINPT), average internode length (AINL), number of roots per plant (NRPP), root length per plant (RLPP), leaf to stem ratio (LSR) and dry matter yield (DMY)) and chemical compositions of the forage were recorded using the standard methods. About 500gm of Brachiaria Hybrid Mulato II grass were collected at each harvesting day from each plant spacing. The samples were dried at 65oc for 72 hours and ground to pass through a 1mm sieve then after grounded the sample taken for analyses of chemical compositions of the forage such as dry matter content (DM); ash content, neutral detergent fiber content (NDF), acid detergent fiber content(ADF), acid detergent lignin content(ADL), crude protein content (CP), crude protein yield (CPY). All the morphological, forage yield and chemical composition data were subjected to General Linear Model (GLM) ANOVA procedures of SAS statistical computer package version 9.0 and Pearson correlation analysis, with significance tested at p < 0.05. The result of the current study showed that, main factors (harvesting day and plant spacing) interaction effect was significant(P<0.05) on NTPP, NLPP, NNPT, NRPP (P<0.001), DMY (P<0.001) and chemical composition of DM, OM, Ash, CPY NDF, ADE, ADL (P<0.001) and CP (P,0.01) but not (P>0.05) in PH, LLPP, AINL, LSR, and RLPP. Morphological characteristics of PH, NTPP, NLPP, LLPP, NINPT, AINL, NRPP, RLPP, DMY and chemical compositions of DM, OM, CPY, NDF, ADF and ADL were increased in advance of plant maturity while, LSR, Ash and CP content were declined trend with harvesting day increased. Maximum DMY (16.55 t/ha) and CPY (1.86 t/ha) were recorded from late (105) day of harvest. The recorded CP content 17.73 at 75, 14.39 at 90 and 11.38 at 105 harvesting day were exceeded the requirement of CP (7.0%) rumen microbial functioning. The highest NDF content (64.11%) was observed at late (105) harvesting. The highest DM (94.53%), DMY (14.28 t/ha) and CPY (1.91 t/ha) were recorded at narrow (20cm) plant spacing. On the other hand, space between plant decreased progressively decreased the content of NDF, ADF and ADL. From the study, it is concluded that late (105) harvesting day and narrow (20cm) plant spacing recorded for optimum CP and DM yield of Brachiaria hybrid cv. Mulato II grass and it can be recommended for forage production in areas having similar condition to the studied area. Keyword: Chemical compositions, Forage Characteristics, Harvesting Day, Plant Spacing, Yield,