Background and Objectives: Neutropenic fever (NF) is a major cause of mortality and morbidity in patients undergoing hematopoietic stem cell transplantation (HSCT). To date, no study has discussed the relationship of fever days in HSCT with the time between recording the fever and administering antibiotics. This study aimed to examine the association between fever days in HSCT and the time interval between recording the fever and intravenous (IV) antibiotics to the febrile neutropenia patient. Materials and Methods: A total of 22 patients who developed NF after HSCT in one hospital were analyzed. Patients who received IV antibiotics injection within 30 min were categorized in group A and those who received the injection after 30 min were categorized in group B. Fever was defined by an attack with an oral temperature of 38.3 °C. Patients’ characteristics and possible risk factors were recorded and analyzed. Results: Groups A and B had 14 and 8 patients, respectively. Patient characteristics, including age, diagnosis, sex, and antibiotics level, were similar between the two groups. The median duration of fever days was 1.5 (range, 1–5) in group A and 6.5 (range, 1–14) in group B (p = 0.003). Multivariant analysis of possible independent impact factors of “fever days in HSCT” was performed. The odds ratio of “antibiotics given time” was 4.00 (95% confidence interval [CI] = 2.26 to 7.22, p = 0.001). The “antibiotics level” did not affect the NF period (odds ratio = −0.80, 95% CI = −2.40 to 1.07, p = 0.453). Conclusions: Rapid IV administration of antibiotics (<30 min after fever attack) can reduce the fever days in patients undergoing HSCT.