Soil respiration is a major pathway of flux in the terrestrial ecosystem carbon cycle and has received a great deal of attention recently. This is largely because even a small change in soil respiration can exacerbate the annual input of CO2 into the atmosphere. Soil respiration represents the combined respiration of roots and soil micro- and macro-organisms, and a number of studies on soil respiration have been undertaken in a variety of ecosystems. In spite of these, our understanding on the mechanisms of soil respiration is not fully achieved largely due to the many complex reactions involved in soil ecosystems. This study aimed to measure a seasonal soil CO2 efflux and compare their values in three old-growth and recently regenerating forests in the White Mountains of New Hampshire during growing season in 2008 - 2009. Seasonal patterns of soil CO2 efflux at both sites showed a high correlation with soil temperature at 10 cm depth. We would expect big different rates of soil CO2 efflux at the old sites due to deeper soil layer and greater biological activity, but some sites were not significantly higher (p < 0.05). Fine roots biomass ( > 5 mm) were higher in old forests but litterfall biomass were no difference between young and old forests. Therefore, further studies about more factors and continuous measurements that are affected to soil respiration including climate, vegetation, substrate quality, net ecosystem productivity and others are needed.