CCD photometry for stars in three fields centered on the old open cluster NGC 188 is presented, with the aim of investigating the HR diagram distribution of cluster main-sequence stars. A sequence of subgiant, turnoff, and main-sequence stars has been detected, extending the observed lower main sequence down to about m(v) = 20 mag. It is found that the observed color-magnitude diagram appears well fitted by a theoretical isochrone for an age of about 6-billion yr, provided that Yale transformations from the theoretical into the observational plane are assumed. From the distribution of the stellar luminosities, it is found that the turnoff stars should have masses of about 1.3 solar mass. The cluster evolutionary scenario is discussed in connection with the reported luminosity of the clump of He-burning giants.