Astaxanthin has been widely used as a feed supplement in poultry and aquaculture industries. One challenge for astaxanthin production in bacteria is the low percentage of astaxanthin in the total carotenoids. An obligate methanotrophic bacterium Methylomonas sp. 16a was engineered to produce astaxanthin. Astaxanthin production appeared to be dramatically affected by oxygen availability. We examined whether astaxanthin production in Methylomonas could be improved by metabolic engineering through expression of bacterial hemoglobins. Three hemoglobin genes were identified in the genome of Methylomonas sp. 16a. Two of them, thbN1 and thbN2, belong to the family of group I truncated hemoglobins. The third one, thbO, belongs to the group II truncated hemoglobins. Heterologous expression of the truncated hemoglobins in Escherichia coli improved cell growth under microaerobic conditions by increasing final cell densities. Co-expression of the hemoglobin genes along with the crtWZ genes encoding astaxanthin synthesis enzymes in Methylomonas showed higher astaxanthin production than expression of the crtWZ genes alone on multicopy plasmids. The hemoglobins likely improved the activity of the oxygen-requiring CrtWZ enzymes for astaxanthin conversion. A plasmid-free production strain was constructed by integrating the thbN1-crtWZ cassette into the chromosome of an astaxanthin-producing Methylomonas strain. It showed higher astaxanthin production than the parent strain.