The effects of variation in average nanofibers diameter and surface density of nanofibers web on thermal conductivity of poly(methyl methacrylate) (PMMA) nanofibers web are investigated in this paper. With an increase in the concentration of PMMA in the solution from 18 to 26 w/v%, the mean diameter of nanofibers increased from 73 +/- 16 to 968 +/- 97 nm. The results demonstrate that the use of thinner PMMA nanofibers results in a lower limit of thermal conductivity and better thermal insulation performance that means lower radiative conductivity. It can be related to a higher specific surface of thinner PMMA nanofibers that means more surface area for radiative scatter and absorption lead to lower conductivity. With increasing surface density of PMMA nanofibers web, the thermal conductivity was reduced and water vapor transmission rate and thermal insulation ability were improved. Higher porosity percentage and smaller pore sizes in the web with higher surface density could result in lower thermal conductivity.