Assessment of Motivation to learn (MtL) is challenging in the diverse populations. Commonly used method (i.e., exact invariance test) may not be suitable for companions of many subgroups. This study employed an alignment optimization method (a.k.a., approximate invariance test) and data of the working age (25-65 years old) U.S. adults (n = 5,509) from the 2012/2014 Program for International Assessment of Adult Competencies (PIAAC) to examine the existing four-item MtL measure across sixteen subgroups by four age-groups, college education (vs. less than college) and literacy levels (low vs. mid-high). Four-item MtL measure was valid. Higher education and mid-high literacy were consistently linked to higher MtL regardless of age. Also, the MtL ranking of subgroups was provided for policy implications.