Dengue outbreak in Siwan district in 2018 was investigated to determine the causes of current outbreak and prevent occurrence of outbreak in future. One patient from Hariharpur Lalgarh Village of Siwan was Zika Positive at SMS Hospital, Jaipur. Both Dengue & Zika virus is transmitted by same vector Aedes aegypti. The urban areas of Siwan like Puranikila, Siwan Nagar, Dakhin Tola, Sukultola, saikhtola, kaghzi mohalla had reported increased number of fever cases & confirmed Dengue positive in Microbiology laboratory of PMCH, Patna. Entomological findings confirmed the presence of Aedine vector in the affected area. Finding confirms the factor leading to the present outbreak and highlights risk factor & control strategies to prevent future in the Siwan district. Community support and participation is also crucial for the prevention of future outbreaks and improving the health and well being of population in the Siwan district.