We present results based on the observations of the blazar 3C 66A from 2005 November 06 to 2016 February 14 in the BVR and I broadbands using 1.2m telescope of the Mt .
Abu InfraRed Observatory ( MIRO ) .
The source was observed on 160 nights out of which on 89 nights it was monitored for more than 1 hr to check for the presence of any intra-day variability ( IDV ) .
The blazar 3C 66A exhibited significant variations in the optical flux on short and long term time scales .
However , unlike highly variable S5 0716+71 , it showed IDV duty cycle of about 8 % only .
Our statistical studies suggest the IDV time scales ranging from \sim 37 min to about 3.12 hours and , in one case , a possibility of the quasi-periodic variations with characteristic timescale of \sim 1.4 hrs .
The IDV amplitudes in R - band were found to vary from 0.03 mag to as much as 0.6 mag , with larger amplitude of variation when the source was relatively fainter .
The typical rate of the flux variation was estimated to be \sim 0.07 mag hr ^ { -1 } in both , the rising and the falling phases .
However , the rates of the brightness variation as high as 1.38 mag/hr were also detected .
The shortest timescale of the variation , 37 min , sets an upper limit of 6.92 \times 10 ^ { 14 } cm on the size of the emission region and about 3.7 \times 10 ^ { 8 } \mathrm { M } _ { \odot } as an estimate of the mass of the black hole , assuming the origin of the rapid optical variability is in close vicinity of the central SMBH .
The long-term study suggests a mild bluer-when-brighter behavior , typical for BL Lacs .