Hydrocarbon fluid inclusions (HCFIs) in mineral grains of source, reservoir, or carrier rocks are of particular interest to the petroleum industry. The minute size of HCFIs warrants a combination of microscopy and spectroscopy for identification and characterization. This article presents fluorescence emission data of pure petroleum oils of known American Petroleum Institute's gravities (APIG) and proposes an empirical tool for predicting the APIG of oils in micron-sized HCFIs through a noninvasive, nondestructive procedure using microscopy-based fluorescence estimates. It also documents how this empirical tool could be used as a way of inferring the APIG in HCFIs by considering the samples from RV-1 well (Mumbai offshore basin, India) as an example. RV-1 is a nonproducing well from the Mumbai offshore basin, India with proven commercial productivity. Fluorescence emission of 13 crude oil samples with known API gravities were recorded using a diode laser excited at 405 nm in order to estimate the API gravity values of oils trapped as HCFIs. The gathered fluorescence data were fitted in a Gaussian distribution to identify unique emission peaks (i.e., around 500, 560, and 620 nm). Bivariate scatterogram with a smooth line fit using spectral ratio at F620/F560 versus API gravity of crude oil provided a classificatory scheme or an API gravity predictor of oils in hydrocarbon fluid inclusions. Fluorescence emission of oil in HCFIs were recorded in the region of 406-720 nm using the microscopy-based fluorescence technique, and the calculated spectral emission ratios at F620/F560 are given in the scatter plot of API gravity known oils. The APIG of unknown samples (whether they be HCFI's or otherwise) can be inferred from the algebraic expression linking emission spectra to APIG for known crude oil samples. The methodology developed is reliable in deriving an accurate API (by reference to the calibration of crude oils) so as to estimate the API gravities of minute-sized oil samples in HCFIs and therefore could prove to be a useful tool in the petroleum exploration and industry.